Update: Made a similar PR for platform-ststm32: Add Mbed CE support for STM32 parts by multiplemonomials · Pull Request #863 · platformio/platform-ststm32 · GitHub
Also, I realized that the framework-mbed-ce version I submitted had a dumb mistake which caused it to not work on Linux/Mac (hardcoded a path with backslashes). So I just uploaded version 6.99.2 which fixes this. (We will eventually be doing “real” versions but not yet).