Thanks! I've definitely considered MIDI out and the remaining modes that the Arduinoboy supports, I just haven't gotten around to implementing it.
At the moment I've extended the necessary GPIO pins to a small breadboard, and unsoldered a link port from a GB mainboard that I broke earlier this year, so I wouldn't have to cut the link cable.
The code is hosted at GitHub, but it is minimal at the moment.
I want to develop it further, and it would be really nice to work together with someone, but the problem is that I can't make any commitments right now due to my studies. Maybe I'll have more time after New Years. If you can live with that, I'd be happy to work together with you. 