Instead of having to modify the inner mechanism to fit in the bank of
solenoids, would it be easier to built a removable unit that would fit
over the keyboard and press the keys?

Actually, the concept is quite similar to that of the early "push up"
piano players in which mechanical "fingers" pressed the keys.  Just
a thought...

A quick question: many MIDI files are in 88-note piano format.  Would
you be using pipe organ MIDI files?

Robert Pelletier