An NFS proxy running over TCP (as opposed to UDP) socket
connections is described in the Bellovin and Cheswick book
(the bible of this mailing list) "Firewalls and Internet Security".
And as with the other services described in the book, they
developed, implemented and use it at AT&T Bell Labs.
I don't think that they give it away though (it may have unfreed
AT&T source code in it). The list of net resources in the back
points the reader at the Linus source code's user level NFS
server as a good place to begin writing an NFS proxy.