Actually, I've done it. I multi-regged using an early PMS screw-in tank, a Female Stab, and a full-size Rock sticking out of that. I don't know exactly what pressure I was feeding into the 'brush (an old Badger double-action) but I suspect it was 40-50 psi.
I forget where I got it, it might have been a factory Badger part, but I happened to have a 10-32/Badger-hose fitting, so it was all a plug-and-play job.
CO2 tanks chill quickly. A 20-lb bulk tank lasts pretty well, but the fittings start to develop a lot of frost after only a few minutes. A 20-oz tank freezes pretty quickly (15-20 minutes, maybe?) and a mostly-full 3K/68 doesn't last much longer.
I'd wager a good fill in a 68 or 72 ci 4500 tank might last 30 minutes or better, depending on your brush pressure and coverage.
On a similar idea, years ago I had to get some parts from a wrecking yard, including cutting some sheetmetal. I took a 20-lb bulk CO2 tank, to which I'd hooked an old-style Max Flow regulator. To that I hooked a regular shop air compressor hose, and used the whole thing to run an air cutoff wheel.
While I got the job done, I used at least a third of the tank, maybe closer to half, and cooled the whole mess down to about 40 psi- and it wasn't that big a job or that thick a sheetmetal.