Hudson Bay blankets are great. We had a cabin in Michigan's U.P. that had passive floor heat (i.e. warm enough to keep the pipes from freezing, but that's it). Each bedroom had two HB blankets, and staying warm was no problem. They are on the list for home use, definitely.

I think for the trunk of the car I may get a couple of these:
http://beprepared.com/product.asp_Q_pn_E_CW%20B100_A_name_E_Wool+Blend+Blanket

They look like prison specials, but at $14 each I won't cry if they get messed up.

BTW, I have overnighted with the gear above, although in fall rather than true winter conditions. It was great. I call it a "10 essntials hike," and I want to do another one this spring. There is nothing like practice to make you carefully consider your gear choices. I carry more than most folks around here on a dayhike, but I think they might reconsider if they had to bivy with it once or twice.