Ouh! Thanks Tom, I'm about to look at that.

SSH can be toggled on as you suggested. That was by adding an add-on which runs once. You also need to enable ROOT_SSH to be able to actually connect to it with a terminal - no other user accounts can be used to log in without further changes (which require root connection).

So, while SSH is enabled and rysnc is enabled, there's normally no UI bits in the web interface to tell the backup process to use SSH (and a direct to shell connection) instead of name/password and a connection using a module via definition in the remote rsyncd.conf