Put the annex objects in .git/annex/objects/ inside the export remote.
This way, when importing from the remote, they will be filtered out.
Note that, when importtree=yes, content identifiers are used, and this
means that pushing to a remote updates the git-annex branch. Urk.
Will need to try to prevent that later, but I already had a todo about
that for other reasons.
Untested!
Sponsored-By: Brock Spratlen on Patreon