Tea strainer experiment didn't work, this is after about an hour. I couldn't shake it too aggressively as its only a tiny tea strainer but clearly I didn't do enough.
I did speak to the roaster who does this a few hundred litres at a time, he said he throughly grades the grounds with a 200 mesh screen and then once it's brewed, runs it through the same size stainless mesh to separate the liquid. For reference, the tea strainer I used is probably closer to a 25 mesh so no wonder it was so wasteful. The pore size of a paper filter is 20 microns apparently, which would be about a 1300 mesh (if it went that high). No wonder I'm having so much trouble with it.
Also, I bought some more N2 cannisters, they're bigger and don't fit the holder on the cream whipper.
It looks like ISI want you to buy their special "Nitro" dispenser. I'll probably machine an adapter when I'm home.