Had a 'Duh!' moment the other day concerning automated backups and allowing teachers/admins access to the alternative directory.
This may or may not work for you but ...
in moodledata/repository create manually a folder called 'ab'. In config of automated backups point to the path to the alt directory to /pathtomoodledata/repository/ab/
In a system admins only course, set up a file system repo for 'restores' to point to the 'ab' repo.
There are +'s and -'s ... any course in which you setup the 'restores' file system repo will have access to *all* the backups. But, if/when one does a site backup and archives the moodledata directory, one is also getting the auto backups of all courses in that archive (but then again, that could be rather large).
If one desires to restrict access to ALL auto backups in 'ab' it's a little more work to build a restricted (like teacher only) repo, but the backups can be copied from ab to the 'teacher' repo.
Yin and Yang!
'spirit of sharing', Ken