Not Bluetooth, but tempting to give this idea a try using a sub-$1 microcontroller [0] implementing a software-defined radio with a PCB trace antenna which is powered from the kinetic energy when you actually hit the key.
Not bluetooth, but every key being wireless is great for ergonomic keyboards. You can tune the layout iteratively without soldering anything. I think there's enough kinetic energy for a short transmit each time the key is pressed, but currently the energy is now dispersed to bounce the key back up. A thick keyboard (like kinesis or dactyl) does have space for a CR1616 battery under each switch though.. That with an NFC/RFID chip for each key might actually enable a truly wireless keyboard.
Put in a mini display to the key, and customise your keycaps! See my other comment about the EspUSB and trying to type other languages.
The ARM core wouldn't be ideal for that though, because it would need a separate WiFi chip. Probably better just to use an ESP (ESP8266, ESP8285, ESP32-C3) that has WiFi (sometimes also BLE) on the silicon.
For an easier challenge, I wish there's a TOMU that sits flush with the USB port, and with functioning push buttons on the side. Then I could add 2 extra keys to my keyboard!