メモ。
Unityフォーラムからのコピペ。
Adjustable Character Spacing - free script
新しいuGUIでは、文字間の調整はできないみたいで、上のフォーラムでは、テキストに適用するスクリプトを親切な人が書いてくれてる。
Unityのバージョンごとで、仕様が違うみたいで、上のフォーラムでは、Unityのバージョンが上がる度に、スクリプトを修正してくれてる。
以下、5.3.3で動かせたスクリプト（転載元）。
Assetsにインポートして、テキストにドラッグして、コンポーネントとして追加すれば使える。
using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using System.Text.RegularExpressions;

/*
http://forum.unity3d.com/threads/adjustable-character-spacing-free-script.288277/
Unity 5.1 and 5.2.2+ compatible
Produces an simple tracking/letter-spacing effect on UI Text components.
Set the spacing parameter to adjust letter spacing.

Negative values cuddle the text up tighter than normal. Go too far and it'll look odd.
Positive values spread the text out more than normal. This will NOT respect the text area you've defined.
Zero spacing will present the font with no changes.
Relies on counting off characters in your Text component's text property and
matching those against the quads passed in via the verts array. This is really
rather primative, but I can't see any better way at the moment. It means that
all sorts of things can break the effect...

This component should be placed higher in component list than any other vertex
modifiers that alter the total number of verticies. Eg, place this ABOVE Shadow
or Outline effects. If you don't, the outline/shadow won't match the position
of the letters properly. If you place the outline/shadow effect second however,
it will just work on the altered vertices from this component, and function
as expected.

This component works best if you don't allow text to automatically wrap. It also
blows up outside of the given text area. Basically, it's a cheap and dirty effect,
not a clever text layout engine. It can't affect how Unity chooses to break up
your lines. If you manually use line breaks however, it should detect those and
function more or less as you'd expect.

The spacing parameter is measured in pixels multiplied by the font size. This was
chosen such that when you adjust the font size, it does not change the visual spacing
that you've dialed in. There's also a scale factor of 1/100 in this number to
bring it into a comfortable adjustable range. There's no limit on this parameter,
but obviously some values will look quite strange.

Now component works with RichText. You need to remember to turn on RichText via the checkbox (text.supportRichText)
and turn on component's [useRichText] checkbox.
*/

namespace UnityEngine.UI
{
	[AddComponentMenu("UI/Effects/Letter Spacing", 15)]
	#if UNITY_5 && !UNITY_5_0 && !UNITY_5_1
	public class LetterSpacing : BaseMeshEffect
	#else
	public class LetterSpacing : BaseVertexEffect
	#endif
	{
		private const string SupportedTagRegexPattersn = @"<b>|</b>|<i>|</i>|<size=.*?>|</size>|<color=.*?>|</color>|<material=.*?>|</material>";
		[SerializeField]
		private bool useRichText;

		[SerializeField]
		private float m_spacing = 0f;

		protected LetterSpacing() { }

		#if UNITY_EDITOR
		protected override void OnValidate()
		{
			spacing = m_spacing;
			base.OnValidate();
		}
		#endif

		public float spacing
		{
			get { return m_spacing; }
			set
			{
				if (m_spacing == value) return;
				m_spacing = value;
				if (graphic != null) graphic.SetVerticesDirty();
			}
		}

		#if UNITY_5 && !UNITY_5_0 && !UNITY_5_1

		/**
        * Note: Unity 5.2.1 ModifyMesh(Mesh mesh) used VertexHelper.FillMesh(mesh);
        * For performance reasons, ModifyMesh(VertexHelper vh) was introduced
        * @see http://forum.unity3d.com/threads/unity-5-2-ui-performance-seems-much-worse.353650/
        */
		public override void ModifyMesh(VertexHelper vh)
		{
			if (!this.IsActive())
				return;

			List<UIVertex> list = new List<UIVertex>();
			vh.GetUIVertexStream(list);

			ModifyVertices(list);

			vh.Clear();
			vh.AddUIVertexTriangleStream(list);
		}

		public void ModifyVertices(List<UIVertex> verts)
		{
			if (!IsActive()) return;

			Text text = GetComponent<Text>();
			if (text == null)
			{
				Debug.LogWarning("LetterSpacing: Missing Text component");
				return;
			}

			string[] lines = text.text.Split('\n');
			Vector3 pos;
			float letterOffset = spacing * (float)text.fontSize / 100f;
			float alignmentFactor = 0;
			int glyphIdx = 0;  // character index from the beginning of the text, including RichText tags and line breaks

			bool isRichText = useRichText && text.supportRichText;
			IEnumerator matchedTagCollection = null; // when using RichText this will collect all tags (index, length, value)
			Match currentMatchedTag = null;

			switch (text.alignment)
			{
			case TextAnchor.LowerLeft:
			case TextAnchor.MiddleLeft:
			case TextAnchor.UpperLeft:
				alignmentFactor = 0f;
				break;

			case TextAnchor.LowerCenter:
			case TextAnchor.MiddleCenter:
			case TextAnchor.UpperCenter:
				alignmentFactor = 0.5f;
				break;

			case TextAnchor.LowerRight:
			case TextAnchor.MiddleRight:
			case TextAnchor.UpperRight:
				alignmentFactor = 1f;
				break;
			}

			for (int lineIdx = 0; lineIdx < lines.Length; lineIdx++)
			{
				string line = lines[lineIdx];
				int lineLength = line.Length;

				if (isRichText)
				{
					matchedTagCollection = GetRegexMatchedTagCollection(line, out lineLength);
					currentMatchedTag = null;
					if (matchedTagCollection.MoveNext())
					{
						currentMatchedTag = (Match)matchedTagCollection.Current;
					}
				}

				float lineOffset = (lineLength - 1) * letterOffset * alignmentFactor;

				for (int charIdx = 0, actualCharIndex = 0; charIdx < line.Length; charIdx++, actualCharIndex++)
				{
					if (isRichText)
					{
						if (currentMatchedTag != null && currentMatchedTag.Index == charIdx)
						{
							// skip matched RichText tag
							charIdx += currentMatchedTag.Length - 1;  // -1 because next iteration will increment charIdx
							actualCharIndex--;						  // tag is not an actual character, cancel counter increment on this iteration
							glyphIdx += currentMatchedTag.Length;	  // glyph index is not incremented in for loop so skip entire length

							// prepare next tag to detect
							currentMatchedTag = null;
							if (matchedTagCollection.MoveNext())
							{
								currentMatchedTag = (Match)matchedTagCollection.Current;
							}

							continue;
						}
					}

					int idx1 = glyphIdx * 6 + 0;
					int idx2 = glyphIdx * 6 + 1;
					int idx3 = glyphIdx * 6 + 2;
					int idx4 = glyphIdx * 6 + 3;
					int idx5 = glyphIdx * 6 + 4;
					int idx6 = glyphIdx * 6 + 5;

					// Check for truncated text (doesn't generate verts for all characters)
					if (idx6 > verts.Count - 1) return;

					UIVertex vert1 = verts[idx1];
					UIVertex vert2 = verts[idx2];
					UIVertex vert3 = verts[idx3];
					UIVertex vert4 = verts[idx4];
					UIVertex vert5 = verts[idx5];
					UIVertex vert6 = verts[idx6];

					pos = Vector3.right * (letterOffset * actualCharIndex - lineOffset);

					vert1.position += pos;
					vert2.position += pos;
					vert3.position += pos;
					vert4.position += pos;
					vert5.position += pos;
					vert6.position += pos;

					verts[idx1] = vert1;
					verts[idx2] = vert2;
					verts[idx3] = vert3;
					verts[idx4] = vert4;
					verts[idx5] = vert5;
					verts[idx6] = vert6;

					glyphIdx++;
				}

				// Offset for carriage return character that still generates verts
				glyphIdx++;
			}
		}
		#else
		public override void ModifyVertices(List<UIVertex> verts)
		{
		if (!IsActive()) return;

		Text text = GetComponent<Text>();
		if (text == null)
		{
		Debug.LogWarning("LetterSpacing: Missing Text component");
		return;
		}

		string[] lines = text.text.Split('\n');
		Vector3  pos;
		float    letterOffset    = spacing * (float)text.fontSize / 100f;
		float    alignmentFactor = 0;
		int      glyphIdx        = 0;

		bool isRichText = useRichText && text.supportRichText;
		IEnumerator matchedTagCollection = null; //when using RichText this will collect all tags (index, length, value)
		Match currentMatchedTag = null;

		switch (text.alignment)
		{
		case TextAnchor.LowerLeft:
		case TextAnchor.MiddleLeft:
		case TextAnchor.UpperLeft:
		alignmentFactor = 0f;
		break;

		case TextAnchor.LowerCenter:
		case TextAnchor.MiddleCenter:
		case TextAnchor.UpperCenter:
		alignmentFactor = 0.5f;
		break;

		case TextAnchor.LowerRight:
		case TextAnchor.MiddleRight:
		case TextAnchor.UpperRight:
		alignmentFactor = 1f;
		break;
		}

		for (int lineIdx = 0; lineIdx < lines.Length; lineIdx++)
		{
		string line = lines[lineIdx];

		int lineLength = line.Length;

		if (isRichText)
		{
		matchedTagCollection = GetRegexMatchedTagCollection(line, out lineLength);
		currentMatchedTag = null;
		if (matchedTagCollection.MoveNext())
		{
		currentMatchedTag = (Match)matchedTagCollection.Current;
		}
		}

		float lineOffset = (lineLength -1) * letterOffset * alignmentFactor;

		for (int charIdx = 0, charPositionIndex =0; charIdx < line.Length; charIdx++, charPositionIndex++)
		{
		if (isRichText)
		{
		if (currentMatchedTag != null && currentMatchedTag.Index == charIdx)
		{
		// skip rich text tag matched (-1 because each iteration already increments the indices)
		charIdx += currentMatchedTag.Length - 1;
		glyphIdx += currentMatchedTag.Length - 1;
		charPositionIndex--;
		currentMatchedTag = null;
		if (matchedTagCollection.MoveNext())
		{
		currentMatchedTag = (Match)matchedTagCollection.Current;
		}
		}
		}

		int idx1 = glyphIdx * 4 + 0;
		int idx2 = glyphIdx * 4 + 1;
		int idx3 = glyphIdx * 4 + 2;
		int idx4 = glyphIdx * 4 + 3;

		// Check for truncated text (doesn't generate verts for all characters)
		if (idx4 > verts.Count - 1) return;

		UIVertex vert1 = verts[idx1];
		UIVertex vert2 = verts[idx2];
		UIVertex vert3 = verts[idx3];
		UIVertex vert4 = verts[idx4];

		pos = Vector3.right * (letterOffset * charPositionIndex - lineOffset);

		vert1.position += pos;
		vert2.position += pos;
		vert3.position += pos;
		vert4.position += pos;

		verts[idx1] = vert1;
		verts[idx2] = vert2;
		verts[idx3] = vert3;
		verts[idx4] = vert4;

		glyphIdx++;
		}

		// Offset for carriage return character that still generates verts
		glyphIdx++;
		}
		}
		#endif

		private IEnumerator GetRegexMatchedTagCollection(string line, out int lineLengthWithoutTags)
		{
			MatchCollection matchedTagCollection = Regex.Matches(line,SupportedTagRegexPattersn);
			lineLengthWithoutTags = 0;
			int tagsLength = 0;

			if (matchedTagCollection.Count > 0)
			{
				foreach (Match matchedTag in matchedTagCollection)
				{
					tagsLength += matchedTag.Length;
				}
			}
			lineLengthWithoutTags = line.Length - tagsLength;
			return matchedTagCollection.GetEnumerator();
		}
	}
}