Follow this, approximately https://medium.com/@alwint3r/working-with-seeed-xiao-ble-sense-and-platformio-ide-5c4da3ab42a3
But in order to get platformio to not overwrite that package, first build the project with the write board target; allow platformio to download framework-arduino-mbed (version ~3.1.1 for me); then paste the specified package over it, overwriting everywhere. That happened to work.
If having flashing problems, be sure to close everything that could be occupying serial ports -- including Cura, Arduino agent!