AFAICT, the fall-through idea is really interesting but may get
problematic when the lower-level file system is modified. The original
union mount code should be able to handle readdir (after a new opendir)
and lookup even if the lower file system is bind-mounted to another
location and updated concurrently.
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.