The Ensoniq "PolyKey" keyboard uses a non-contact inductive system to handle initial velocity and then polyphonic aftertouch. All thats on each (sprung) key is a lump of metal.

Regrettably the reliability of the keyboard in the field was compromised for some time by dodgy PCB interconnects (there are two PCBs handling halves of the keyboard)(although my SD1 was fine) and the key feel is firm but considered very "clunky" by some people.

Polyphonic aftertouch is however fabulous for big-band stuff; your ensemble playing remains untouched while your solo can go mad with aftertouch triggered vibrato!

Back to the plot - assuming the sensors will be using LED light sources could they could use infra-red or ultra-violet LEDs?