So, the last thing I wasn't doing was erasing the chip before flashing; it has to be done in a separate step, and I didn't know that. So, the project todo list now looks like: 

* get µpython to talk over the USB OTG

* write some kind of frambuffer driver accessable from python (possibly Cube?)

* build out a watchface

* design/build circuit PoC (maybe with the LGA144 footprint for ease-of-soldering)

* design/build actual wearable circuit

* build case

* party