For key sensor, it says "非接触 連続検出 電磁誘導センサー" which Google translates to "Non-contact continuous detection electromagnetic induction sensor".
Probably something simpler than RFID. Based on the picture and the requirements, you put some sort of little inductive loop on the under side of the key stick, and then design the most inductive thing you can out of PCB traces, that hopefully directs the magnetic field straight up(?). Then constantly measure the inductance of whatever you built on the PCB. It'll change with distance.
You get a lot more design options when you're Yamaha and can design/fab ASICs. 😔