^ Thanks, it's starting to come back to me. I actually did the wiring for the electrics on my furnace...it was 20+ years ago and the memory ain't getting any better...

IIRC there's a 24V-operated gas valve in there. It needs to have power to stay open (partly a safety feature). I can manually switch the fan off, so yeah, getting 24VAC to the furnace at the right place at a minimum would be needed to keep it going. With the fan switched off, perhaps even a 12V (car) battery connected to a smallish inverter whose output is jury-rigged to the (isolated) 120VAC furnace input could work.

I am comfortable working with electric stuff, it's my job (but furnaces aren't!), so I can do things relatively safely, please ignore this stuff if you don't follow it exactly. You obviously shouldn't screw with gas furnaces ever, but in a serious emergency it's good to know an option...