So, I've got mechanical hardware that will fit: The framework one-key modules are designed to mount to a PCB, and I can design and fabricate PCBs. So I next gave some thought to the electrical planning.

I know the Framework keyboard uses an RP2040 microcontroller - from the Raspberry Pi Pico. I've never used those (my standard choice is STM32 chips) but I figure it makes sense to use the RP2040 for consistency with the other Framework hardware.

The electrical connection to the input modules is USB via a pogo-pin connector and they're held in place by magnets:

The pogo pins provide +5V power and ground, USB and a board ID resistor:

I can create the connector as pads on a PCB, so I'll need to find a way of making the PCB magnetic so it holds in place.