We vacationed in a rental house in Emigrant, MT, for Christmas 2011. The house had about 20 Acres of land with some trees, so I was able to get in a one-night hang... sort of...
HH Expedition A-Sym with Hennessy UQ. (2QZQ zipper mod #4, stock HH rainfly.)
Standard straps, Nitize Figure 9 on each end to allow adjustment.
Blue CCF pad 72" X 20" inside hammock.
North Face Elkhorn synthetic sleeping bag rated for 0* F.
Columbia silver-matrix underwear
Hooded fleece jersey
Merino wool crocheted toque (Daughter made it. Very thick yarn, very bulky.)
Winter coat (Draped over the bottom half of the sleeping bag.)
When I got into my hammock at 9:00 P.M., the temperature was minus 6* F. with a breeze of about 3 MPH. Due to the terrain and snow, I was not able to pitch my rainfly in a very protective manner; it pretty much channeled the (slight) breeze right at me. Still, I was OK.
She Who Must Be Obeyed sent my Son out to drag me inside at about 2:00 A.M. The temperature had dropped to minus 15* F., and the wind had picked up to about 5 MPH with a few gusts in the 10 to 15 MPH range.
I was toasty warm, sleeping like a baby.
The Wife had awakened, checked the temperature, and had tried to call me. I would have been all right except that my cell phone battery had gone dead.
Wife was afraid I had gone hypothermic and could not answer the phone.
Upshot: I believe that, with a superior tarp, my rig could go down to the range of minus 20* F. to minus 25* F..
Also, I believe an OQ could get me there too.
A superior tarp plus an OQ could, I believe, get me down below minus 30*F..
Yellowstone was beautiful in Winter. I recommend a Winter journey to YNP if you get the chance.
You will be glad you did.