I don't promise anything, but I've looking at the schematics. There're some options:
- STM32 pins: PB2+PB10 are free in all boards. But require to solder over these small 0.65mm pins, not for everyone.
- RTC pins: PC14+PC15. All have them, except Ksger v1.5. Wires can be soldered to the caps or crystal pads, they don't need removal.
- EEPROM pins: v1.5 and Quicko: None, v2:PA0+PA1 v3:PA3+PA4. Doesn't need removal, it's extremely hard to randomly match the clock/address sequence.
- Ksger v1.5 has PB12 (BOOT1) routed to a resistor/solder jumper, can be removed without issues.
- Quicko has PA15 routed to the programming header, labeled "RX".
Enabling the internal pull-up resistors, it barely requires any external parts. You could just wire the pin to the sensor, but it's a quick way to destroy the board if something goes wrong.
To prevent for current flowing into the pin, ex. a broken wire touching 24V, use a 1N4148 diode, will withstand 200V and the voltage drop won't be a problem. Then just pull the signal to ground.
For now, let's assume there's only one input.
This is not done by magic, so carefully think what functionalities might worth adding and how to implement them.
Maybe:
WAKE MODE: [STAND] [SHAKE] [SET BY SPECIAL PIN STATE?]