A filesystem "change journal" and other topics

"But this all requires a way to tell what has changed (not only locally, thanks, I want to know if my NFS server gets stuff modified by a client, too, so the server can back it up, yes I know NFS makes this nice and hard: saying "NFSv4 only" is acceptable to me)."

knfsd isn't really *that* special a filesystem user. If the filesystem supported a feature like this, I don't see why we'd have any trouble making it work properly for exported filesystems, and I doubt the protocol would matter.