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Depending on the shot / angle etc that you want and hopefully a "locked of shot"...I would shoot someone playing the piano/tune so you had a "map" of the action. ie know and see what keys are pressed at what time for what duration.
With camera in same place , I would then shoot all the key down /combinations that you need. Manipulate the keys /chords from inside the piano. Basically stop motion but not necessarily in order or sequence.
Lay your map up on a layer
Match shots of chords and keys up on a different layer.
See you back here next year ;-)
PS maybe easier to do in your NLE instead of AEFX