> If you duplicate the entire directory structure in the top level of the
> union, this would not even easily work in the case where you unmount the
> top level, modify the lower level and then recreate the union mount.
Not only that but since you cache the consolidated directory entries on disk, you will not be able to union mount your top layer later on any other lower layer filesystem.
I know that supporting all sorts of corner cases and features has caused major pains for union mount, but this restriction sounds a bit too restrictive to me :)