This needs some simple hardware trigger, that can be queried by bootloader, but specific wnough so it won’t trigger in your pocket. If buttons are unavailable, I can propose several idead:
- Cover the proximity sensor 10 times per second. Requires good motor skills.
- Shine separated red, green, and blue lasers into three of the phone cameras. You’ll need a box with three different-colored LEDs to cover cameras module.
- There is a low-speed data channel in wireless chargers. Just add the button to the charger instead of the phone.
- Put your phone at exactly 2.5G of acceleration. Accelerometer is easy to use even from the bootloader, however you will need to put your phone in a centrifuge and reboot it while it’s rotating.
- An option ‘Reboot into recovery’ in system settings, duh. Won’t help if your ROM fails to boot.
I guess that phone would use eSim, so no slot is needed.