Then DirectInput / XInput, or whatever it's called now should give you access like to any other controller.
In addition, if I remember correctly, some game instruments have Midi output (the latest GH drum kit, the RB Fender guitar and very likely the keyboard), so you can go that route, which would automatically add support for _all_ midi based instruments.
Lastly, why wouldn it be legal to make a game that uses a rock band peripheral? In fact, on XBox you can play pretty much every game with those instruments, it's just not very pleasant. ;)