It should be possible to union mount annexes. So if multiple drives have content, an annex mounting them both would have available all the content from all the drives.

This could be done by just making .git/annex/KEY link to the actual content on the mounted annex.

(Need to make sure the copy tracking code does not confused and think the symlink is a copy of the content.. Also need to make sure that code that writes to .git/annex does not follow symlinks.))