Can't a multi-part performance start at part 2, 3, etc? If I put drums in part 1, and bass in part 2, and then merge in some multi-part performance in parts 3-5, as far as I know I could play that multi-part instrument by setting keyboard input for 3-5. I'd still have to replicate the Super Knob setup for it, but that seems possible to me.

Or am I missing something?