Don't forget the CPU is also a wireless speedo, which is probably what the wheelchair user was using it for.
Current power meters are designed to measure power through the transmission so none would be suitable for a wheelchair. That would need some kind of strain gauge on the wheel grips.
That said, power through the arms would probably be a lot less than the legs, plus there's no consistent flow of torque, more like regular pulses plus lots of freewheeling.