I really like the premise and loved Passpartout The Starving Artist. I That being said I have just have a couple of suggestions:
** With the current layout I found it difficult to make the instruments work well together. I ended up doing "I have a drum here and I know I want to wait a couple of half beats and then...wait what was the next drum beat? <switch to drums> Oh ok <switch to harmonica> got that harmonica note, now for banjo do...wait what was the drum? <switch to drum>" You're basically asking the player to be able to imagine the final product but only being able to look at and work with single slices at once. Like if Passpartout The Starving Artist had a separate canvas for each color and you could only view one canvas at a time.
I was thinking you could try treating the UI to work more like a painting instead of different sheets of music per instrument, so basically keep the existing grid, but treat the music score as a single canvas and "paint" an instrument at a specific note and specific time which covers all instruments except percussion, which you could do with a separate translucent layer that comes up when you hold a key or mouse button or something so that the timing matches up but the context of what happens at that time (the horizontal axis) changes, maybe based on what type of instrument (drums, tambourine, etc)? I don't know, just throwing something out there. In any case though, basically making it so I can see everything that's going to happen at a given time.
** Having the notes next to the little piano keys would be helpful in understanding what's going on without having to purely do it by ear -- without that it's akin to trying to paint a picture with the color picker in grayscale.
On a side note, I'm a fan of music composition and I would love to be part of a beta test/feedback group if you end up doing this! In any case, good luck!