Ooh yeah, you are right to keep it more simple and accessible for this game, I was going off to try and instrumentify the Game Boy, but that would be much better as a separate thing. I don't imagine the changing chords being too unintuitive, take a look at bisonoric accordions which produce a different chord with the same buttons depending on the direction of the bellows.
Anyway, all theoretical here, but I bet you could massage a part of your code from this to make something more for musicians if you'd like.
Thanks for your excitement, it made me happy.