I have it working now. Boot is from an SD card and FreeNAS is an NFS server.
The directory on the server should be owned by root:wheel, and maproot user set to root.
I get only NFSv2 currently, and I think that upgrading to NFSv3 would allow to do more tuning with performance.
I currently have swap through a loop device to a file that resides on the NFS share. Not sure if my Transcend card would perform better or worse. Or I could also use two swaps simultaneously. Except that it is the fastest way to kill the card that I know of.
This Pi was intended to be a lightweight desktop replacement, but Firefox practically kills it. I have the Raspbian default desktop.