I wound up pulling the trigger on an ultralight sleeping bag from Sea to Summit. It weighs 13 oz on my kitchen scale, and in the "lofting case" it comes with, it is smaller than my old sleeping bag in its stuff sack. I've got an emergency bivvy (basically a giant potato-chip bag). I think that if I put on all my clothes, I'll probably be warm enough through the coldest temperatures I'm likely to encounter. Probably. An interesting option I saw was a down jacket + half sleeping bag that just covers the legs, and laces into the hem of the jacket. This would be pretty cool if I thought I would be wearing a down jacket at any other time, but my existing cold-weather kit should be sufficient.
For the TABR, I am not worried about hotels filling up—at least not with other racers. I'd be surprised if 400 people enter the TABR, and the field is extremely spread out. Although if I'll be checking in late (and I will be), it's only polite to make arrangements in advance.
My next problem to solve is figuring out how I want to carry everything. I did a test pack in stuff sacks and my load displaces about 13 liters. There are a lot of ways I could divide that up.
Registration opens in two days.