borg backup is pretty cool, and could be a great special remote backend. In particular it does delta compression and stuff.

There seem to be two ways it could work. Probably there are borg commands that allow storing a given blob in it, and retrieving a given blob. And that could be used for a traditional special remote.

But also, if a whole git-annex repository has been backed up with borg, then git-annex could look inside such a backup, and see if .git/annex/object/ contains an object. It could then mark it as present in the borg special remote. This way you'd use borg to take backups, and git-annex would then be aware of what was backed up in borg, and could do things like count that as a copy.