Portable electronic musical instrument: energy from pressing keys, no batteries: feasible?
Keyboard electronic instrument, self powered? 

I don't think powering from the keypresses will easily work, but there are other possibilities.

I bet you could do something based on a squeezebox! Use a ratchet and gearchain mechanism based on the sort of thing found in those squeeze to power torches and you have plenty of internal volume for a decently efficient speaker. If you can get a toy keyboard with decent battery life off a pair of AA cells, this should be workable.  

Downside is that people will think you are an accordion player.