You could just do what I do and play music over the mic. If you balance the levels correctly (input volume and mic gain) you can achieve great sound. Not to mention its way easier to play on a real music keyboard instead of a qwerty keyboard.
Afaik it isnt against the rules as long as you're doing it in a way it doesnt bother people. I remember putting my mic into my acoustic back in the days of OC and the serious server. Was good fun.