Yes, zero hardware changes or mods to the piano. I think that would be a dealbreaker for many people. It also means that the barrier to entry would be much lower.

Hmm, yes, if you can get it to work with just sound, that would be trivial to set up. If not, I think the light idea would still be valuable, as technically you wouldn't be changing the piano (just adding a camera and light above it).

I've actually toyed around with that idea, one interesting application would be to convert Rousseau style youtube videos back into midi.

I'm quite sure it is possible but it would still require a lot of set-up. The microphone is such a temptingly simple target that it makes sense to pursue that one first.

That makes sense (although the microphone is only a simple target if you ignore the unknown unknown of deconstructing the sound :P). I'm very much looking forward to your progress!