I wonder if megacmd would help here... It is at the end an official client...
@tbone install megacmd for your platform and then use it to run webdav server. In the next step you can connect to it using rclone webdav remote.
There are plenty of examples in the past threads if not clear enough, for example:
Some users also report that megacmd is noticeably faster than mega access implementation in rclone.