[Unionfs] Exporting a union with NFS4 ?

[pascal at pabr.org]

There was a good summary of why unionfs-2.x cannot export unions
over NFS anymore here:
http://thread.gmane.org/gmane.comp.file-systems.unionfs.general/2556

If I understand correctly, it will be hard to restore this
functionality for NFS2/NFS3 (requires ODF ?), but I might be
easier for NFS4 (because it supports volatile file handles ?).

So would it be possible to export unions over NFS4 if we initialized
sb->s_export_op in unionfs_read_super(), along the lines of what was
done in unionfs-1.4 ?

Does anyone have a patch which does that ?  I do not mind if it only
supports lower filesystems with small inode numbers or if branches
cannot be added/removed afterward.

Pascal