This is one of those things that goes to show: it's easy if you know how.

I've got a zfs-based file server (currently using SmartOS) which uses NFSv4 shares. OSX can connect to NFS shares using "Connect To Server" from the finder" using a syntax like this:

nfs://nas.example.com/share_name

I've previously tried to use on my mbp but have never managed to get it to work in a stable fashion.

Then, this evening, I stumbled across the solution:

nfs://vers=4,nas.example.com/share_name

That's all there is to it – I now have stable NFSv4 connections from my Mac!