I installed and tried to use the plugin with the following results:
First of it all, i try to sync to a network share on a NAS, i have only password protected shares.
I use this with calibre on a mac.
When i enter my credentials in your plugin, calibre wont start anymore without errors.
Plugin tells that i have too much ":" in my settings.
I entered the path to my share in this way: "user

asswd@NAS:/share/folder"
Obviously rsync wont work because of the missing password on the NAS...
My second "problem" is that i would send the whole library to the NAS, including the database and all folders inside my library, not only a sort of files. I want to use that rsync-feature as a backup for the whole library...