git annex unused supports an --from option to select a specific remote to find unused data in. When you use that on a repository which is not linked to from the current repository by a git remote, git annex fails:

$ git annex unused --from somerepository
git-annex: there is no available git remote named somerepository
$ git annex unused --from repowithremote
unused repowithremote (checking for unused data...) 
[...]

As git annex at least for newly-deleted files knows when those are stored in the given repository, git annex should at least find those.

done, made it suppose the uuid or description.

Of course, dropunused --from still needs the name of a remote, so this is only useful for querying.. --Joey