Old stuff
From Linux NFS
rfc 5661 includes protocol for directory delegations, but implementation efforts are currently stored; see NFSv4.1 Directory Delegations for experience from an early CITI prototyping effort.
The spkm3 gss mechanism never caught on (hence also lipkey, which was based on it); by the time it was implemented, protocol shortcomings were found, and it was decided a new mechanism would need to be specified. See SPKM3 Issues.
Testing: A now-defunct NFSv4 testing project compiled a bunch of tests that might be worth running on NFSv4, with some results on early code, etc.: