private string Dump(Array data, List<int> indices, int dimension = 0)
{
    string result = "[ ";

    for (var i = 0; i < data.GetLength(dimension); i++)
    {
        if (dimension == data.Rank - 1)
        {
            var temp = data.GetValue(indices.Concat(new int[] { i }).ToArray()).ToString();

            temp = Regex.Replace(temp, @"(Vec[2346][bdfisw] )|([xyz]:)", "");
            temp = Regex.Replace(temp, @"\(", "[");
            temp = Regex.Replace(temp, @"\)", "]");

            result += temp;

            if (i != data.GetLength(dimension) - 1) result += ", ";
        }
        else
        {
            result += Dump(data, indices.Concat(new int[] { i }).ToList(), dimension + 1);                    
            
            if (i != data.GetLength(dimension) - 1) result += ", ";
        }
    }

    return result += " ]";
}

private void Test()
{
    byte[,] array = { { 255, 255, 255, 255, 255, 255, 255, 255 }, 
                      { 255, 255, 255, 255, 255, 255, 255, 255 }, 
                      { 255, 255,   0,   0,   0,   0, 255, 255 }, 
                      { 255, 255, 255, 255, 255, 255, 255, 255 }, 
                      { 255, 255, 255, 255, 255, 255, 255, 255 }, 
    };

    Mat mat = InputArray.Create(array).GetMat();

    mat.GetRectangularArray(out byte[,] data);

    string dump = Dump(data, new List<int>());

    Console.WriteLine(dump);
}

結果：
[ [ 255, 255, 255, 255, 255, 255, 255, 255 ],
[ 255, 255, 255, 255, 255, 255, 255, 255 ],
[ 255, 255, 0, 0, 0, 0, 255, 255 ],
[ 255, 255, 255, 255, 255, 255, 255, 255 ],
[ 255, 255, 255, 255, 255, 255, 255, 255 ] ]