this would be a killer replacement for micro/circuitpython for embedded devices, assuming there's an elegant TS->MQJS transpile

It’s not even the languages or runtimes that inhibit embedded adoption but the software to hardware tooling. Loader scripts, HAL/LL/CMSIS, flashing, etc. They all suck.

It looks like if you write the acceptable MQJS subset of JS+types, then run your code through a checker+stripper that doesn't try to inject implementations of TS's enums and such it should just work?

I don't think you can transpile arbitrary TS in mqjs's JS subset. Maybe you can lint your code in such a way that certain forbidden constructs fail the lint step, but I don't think you can do anything to avoid runtime errors (i.e. writing to an array out of its bonds).