It does run Linux, but I also don't know how its UI passes in its parameters to rsync to be able to make such a script.

I'm hoping someone will make an add-on that enables the SSH option in the UI. That way you can selectively use it, and I won't have any manually made changes to the disk structure zapped when I perform the next update. Someone on their forum is a very prolific add-on maker and has already made the very same thing for the ReadyNAS Duo. Should be a piece of cake for them to make one for the Pro.