tigertrainer:
I thought about MakeyMakey some more, and although it sounds really great, there is one major problem. The musicians can and sometimes do connect their playing sticks to the notes by accident, or when they place their sticks on the instrument while taking a rest (if someone else is playing, but not them). This could add dirty data. It seems the only way to be really clear is to get a volume reading.
If they connect there sticks by accident (I assume producing a note) then no simple system is going to know it's a bum note and not a genuine one.
Resting the stick on the instrument may not be a problem as you could ignore press durations longer than x millisecond if they normally tap it.