API for upload code into device?

Just to add - with https://github.com/espruino/EspruinoTool­s you can output what would have been written to espruino with -o out.js

So you could get your code, run it through that command-line tool, and then use whatever you've got connected to the RX/TX pins to send the bytes from that file - and you've got your firmware update sorted.