How can I output the Alpha channel inside the 24-bit or 18-bit RGB output that would normally go to a display?

You can absolutely do this - its an easy code change. You can do 6 red, 6 blue, 6 green, and 6 alpha bits to make up the full 24, and having 5 red, 5 green, 5 blue and 3 alpha to make 18 bit is as simple as not hooking up the least significant wires on the user end.