I used to carry a northface down mummy bag in my truck, back before I had portable phones and radios, and before most people heard of northface. I think I bought it from campmor. In a plane, I think I'd pack a down mummy bag and a tiny one person 3 season tent. Something that was self-supporting if at all possible.
In the truck, I figure I'd have shelter from the wind and rain, and I'd be toasty warm for a while. That's assuming I was stuck from sliding off the road or the truck breaking down. In an airplane, I'd want to add shelter. My limited experience in fall/winter camping in NY tells me to keep the bag a separate thing from something to keep the wind and snow off you, and that a breathable tent is the way to do that. You don't want a buildup of moisture, and a sealed up waterproof bag would do that. I know there was a thing on goretex bivy sacks for a while, but I never bought into that theory...didn't seem all that much lighter for backpacking, and looked to be a lot less comfortable. Anyway, good bag and lightweight tent, maybe 8-10 lbs for both. That, some water, sat phone or PLB, VHF handheld radio, and some orange cloth for a panel marker, the rest of the 'normal survival stuff' and you should be set, if you end up stuck somewhere.