What a great idea! I designed and built a Theremin using junk box parts
many years ago. Worked pretty well and used 8 or 9 tubes. By design
looked like a Frankenstein's lab creation.

Theremins have two controls; pitch and volume. I could see placing
the 'antennas' under the PF in a way the closer the ball gets to
the flippers, the louder and higher the pitch gets. That'll add another
dimension to EM's or any pin for that matter.

Gonna ponder this one a while..... At one time inexpensive and small
solid state kits were available.
Steve