The API used for the cloud service is quite simple. There are already two projects on github implementing it (one by me) - search for remarkable-tablet to find them.

The tablet also works fine without using the cloud service, it has a built-in webserver to upload documents to it that you can just access via WiFi or USB-LAN.

Really this device is a hacker's dream in terms of openness. The only last thing I'd wish for was open sourcing the main software application running on it.