I got some help with that, and did a little bit of progress, though it still doesn't work the way it should.

People who are smarter than me started to figure things out, and there is a pull request with more documention with some additional discussion that has cleared some stuff for me.

Basically you can only call functions that have been explicitly exposed to the native module mechanism, and if you need some additional functions, they need to be added to a list. Sounds simple enough, and I managed to get that to work, but there is a small catch: it will only work with the firmware that has those functions added as well, so it kind of defeats the whole purpose.

Fortunately, there seems to be some ongoing work on solving this:

• https://github.com/micropython/micropython/pull/5682
• https://github.com/micropython/micropython/pull/5653

So hopefully in a couple of months it will be possible.