Are tubeless mountain bike tire systems appropriate for winter riding?
All of my winter rides are tubeless now.  I have successfully used tubeless setups with Stan's at -40F.  Many other riders in my area have used them extensively and without issue at temperatures well below zero.  Stan's happens to be the fluid used by my LBS, but I am sure there are others that work as well.

The hard part is getting the initial seal to work properly on fat tires.  If they don't fail on the initial ride, they are just as reliable as in warmer temperatures.