I'm sure one of the people who own one will chime in, but i think the whole device is hackable, you can get into a linux shell as root and do whatever you want.
Hopefully this doesn't change, because it's really a pain to put things on the tablet without using their app.
Have you tried Syncthing[0, 1]? I'm actually considering buying a ReMarkable 2 right now and the fact that Syncthing seems to be working on it is playing a major role in my decision process.