I'll tell you one idea I've had that I think would work - you'll think I'm nuts but bear with me...
I have a large-ish american style fridge freezer, firdge one side, freezer the other, and the fridge stopped working, wasn't cooling down. I rang a few repair engineers and one of them said don't bother calling anyone out just empty the freezer and the fridge, then turn it off and put pans of hot water in the freezer side, shut the doors and leave it for 3 or 4 hours.
I did and when I opened it I was amazed that it was still red hot in there, and even more amazing the fridge side was also red hot too - bugger because I'd left the milk in.
Anyway, I digress - these types of fridge freezers work by the coldness from the freezer side going though an air vent and cooling the fridge, and they are absolutely air tight, the amount of heat it retained for hours it was like a thermos flask. So....
If you laid one on it's back, put the N tank in the fridge and open the gas valve and shut the door, then get into the freezer side when it's had time to fill up, and shut the door, I think that would be 100% effective, if perhaps a bit Hammer House of Horror Dracula-esque.