We have pretty good information about the motor shield:
- The H-bridge is TB6612.
- The microcontroller is STM32F030.
- We have a schematic: https://www.wemos.cc/sites/default/files/2016-09/mini_motor.pdf
- We have the Arduino library for talking to it: https://github.com/wemos/WEMOS_Motor_Shield_Arduino_Library
- And, last but not least, we finally have the source code of the original firmware: https://github.com/wemos/Motor_Shield_Firmware
The shield has a serial interface for programming broken out, and that chip is supposed to have a bootloader built-in, so hopefully we will be able to just use stm32flash and an USB2TTL adapter to program it.
There is a nice dev environment template for this family of microcontrollers: https://github.com/szczys/stm32f0-discovery-basic-template