I've ended up using unsafe code and accessing the UnmanagedImage.ImageData directly:
unsafe
{
byte* dst = (byte*)data.Scan0.ToPointer();
for (int y = 0; y < Height; y++)
{
for (int x = 0; x < Width; x++, src++, dst += pixelSize)
{
dst[RGB.A] = input[src].A;
dst[RGB.R] = input[src].R;
dst[RGB.G] = input[src].G;
dst[RGB.B] = input[src].B;
}
dst += offset;
}
}
As it cannot be done in Unity, I had to modify the AForge library.