It turns out that the two mystery pads at one end of the five which are exposed to through the battery bay are Serial Wire Debug lines, or SWD. I have only just heard of this term when I looked into the Mechaduino and how you need the SDW interface for loading the Arduino bootloader on the Atmel SAMD21 that they're using (same chip as in Arduino/Genuino Zero - good call Tropical Labs). I traced the pads ringed in the picture below with the multimeter and the pink one is connected to PA13 (SWDIO) and the red one is connected to PA14 (SWCLK).

I have yet to find out if we can talk to the STM32F via SWD but thought I'd let you know the latest finding. More to follow.

An idea: perhaps there are other chips on this board which have interfaces that would let us talk to the STM32F, like SPI or I2C (although slaves are unlikely to get very far). Datasheets ahoy!