誰もが実装するであろう、BlendShapeのインデックスを登録しておくエディタ拡張です。

MonoBehaviour側

using UnityEngine;
using System.Linq;

public class SetBlendshape : MonoBehaviour
{
    public SkinnedMeshRenderer skin = null;

    public int[] selectedIndexes = Enumerable.Range(0, 7).ToArray();

    private void Start()
    {
    }

    private void Update()
    {
    }
}

エディタ側

using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(SetBlendshape))]
public class SetBlendshapeEditor : Editor
{

    SerializedProperty skinProperty;
    SerializedProperty selectedIndexesProperty;

    private static string[] settings = new string[]
    {
        "A", "B", "C", "D", "E", "F", "G"
    };

    private void OnEnable()
    {
        skinProperty = serializedObject.FindProperty("skin");
        selectedIndexesProperty = serializedObject.FindProperty("selectedIndexes");
    }
    
    public override void OnInspectorGUI()
    {

        var blendShapeNames = GetMeshBlendshapeNames();

        serializedObject.Update();

        EditorGUILayout.PropertyField(skinProperty);

        if (EditorGUILayout.PropertyField(selectedIndexesProperty))
        {
            EditorGUI.indentLevel++;

            for(int i = 0; i < settings.Length; i++)
            {
                BlendNameProperty(selectedIndexesProperty.GetArrayElementAtIndex(i), settings[i], blendShapeNames);
            }

            EditorGUI.indentLevel--;
        }

        serializedObject.ApplyModifiedProperties();
    }


    private string[] GetMeshBlendshapeNames()
    {
        var target = (SetBlendshape)serializedObject.targetObject;

        if (target == null || target.skin == null)
        {
            return null;
        }

        var mesh = target.skin.sharedMesh;
        var blendshapeCount = mesh.blendShapeCount;
        var blendNames = new string[blendshapeCount];

        for (int i = 0; i < mesh.blendShapeCount; ++i)
        {
            blendNames[i] = mesh.GetBlendShapeName(i);
        }

        return blendNames;
    }


    private void BlendNameProperty(SerializedProperty prop, string name, string[] blendShapeNames = null)
    {
        if (blendShapeNames == null)
        {
            EditorGUILayout.PropertyField(prop, new GUIContent(name));
            return;
        }

        var values = new int[blendShapeNames.Length];
        var options = new GUIContent[blendShapeNames.Length];

        for (int i = 0; i < blendShapeNames.Length; ++i)
        {
            values[i] = i;
            options[i] = new GUIContent(blendShapeNames[i]);
        }

        EditorGUILayout.IntPopup(prop, options, values, new GUIContent(name));
    }
}