Tried this, the DMG does not have a full common ground system, sadly. I have used the DMG board as a gamepad board for a GBA (Very close to what you are doing, pretty much just the reverse), and to do it, I had to cut a lot of traces, remove a lot of parts off the board, and wire it all together to make it a common ground setup. Would not work in a DMG, though.
You COULD do what I just said, but hook it up to a GB Pocket or GB Color mobo instead, it would look stock on the outside.
Or, do what I did, and use a GBA SP, as I did in my "Power Gameboy" last year.