Tried myself at it: I created (more like POC-ed) a driver that sees remarkable's cloud as a device and push/pull books from it.
I used
https://github.com/subutux/rmapy as a base to interact with the cloud, and tried to fill all the gaps I could find. But I think calibre don't really expect to interact with devices through the cloud.
Anyway it's full of missing features (the API does not provide authors, file size etc without having to download the file), full of `print` statements and probably full of bugs, but might be enough for simple tasks such as uploading/downloading books.
You'll just need to install a plugin from a zip file and register it through the driver's configuration. (name is Remarkable 2 Cloud Interface, even though I suspect it will work with first gen too)
Here goes the link with the source code and the zip file for the driver:
https://github.com/gautierenaud/rema...bleCloudDriver
Hope it will help!