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This is how: take ESP32 - it has bluetooth, wifi and serial port all integrated and it costs couple of dollars. It's small and it runs on 3.3V, it has a voltage regulator that it allows to connect to 5V.

I can do the code and place the esp32 in a mouse on a Saturday (my C is always rusty, not using it professionally). And I am a hobbyist at best. So the original serial mouse/keyboard are connected to ESP32 that normally proxies the signal of the hand movement to the PC.

If I press a pedal (w/ foot) the second computer would receive a signal, calculate a human alike trajectory to move the mouse and send it to the ES32. The latter will execute it along with the left click to shoot.

Like I said - trivial. The same can be done with the USB port as well, and it's not harder. Just that for PS/2 I have the tools laying around.