Stuart writes to ask if you couldn't put a strap around the wrist to sense the movement of the tendons through the wrist, by way of making a keyless chordite.
I think that would be dangerous. The tendons run inside little tunnels past the carpal bones (hence the name carpal tunnels -- oh these doctors can be SO creative sometimes). Squish the tunnels and you can inflame them. If they get inflamed they can swell up which keeps them inflamed, etc etc etc. Bad news.
No, better to go looking in the muscles further up the arm for myoelectric signals.
But note that it can be more fatiguing to be moving your fingers in mid-air than to be pressing them against something. The Chordite is designed to put the keyswitches at the finger's neutral point, so that relaxation puts your fingers on the keyswitches. It then takes only a little bit of pressure and movement to press the key, but because the key has hysteresis, you can feel it when you have pressed the key.