NFS exporting union mounts is one of my personal top use cases unfortunately. :-) We use the FUSE based unionfs for doing distro mirroring based on ISO files.
We loopback mount all the ISO's, then union these filesystems together to "recreate" the appearance of one tree with everything in it, then are able to export all of this via NFS to clients. We now have the raw .iso files available, each disk individually as well as a merged tree.
This probably isn't the most typical use case for unioning :-) It works, but is rather slow as you might imagine... and our /etc/exports file is very ugly indeed!