Pixracer firmware update without usb access - Bootloader

Hi, I want to build a plane where I have no access to the Pixracer usb port because I want to seal the plane against water. The plane has an RFD868 telemetric link and is connected via telem2 to a companion Raspberry Pi Zero W which can be accessed via wifi. I could also add the ESP8266 wifi module to the Pixracer.

See: https://searchwing.org

Is it in principle possible to do a firmware update without access to USB? I found in the bootloader code that this is possible via UART but from what I understand this is hardcoded to UART2 which is telem1.

Is it required to do real cold start by power cycling to get into the bootloader mode or is there a way to do this by software only means, i.e. software reboot plus possibly setting some parameter?

What would be a good software to handle this update then? I guess QGC is expecting a firmware update via USB?

Some discussion about this was here