Heather
I have used pieces of fur over my face and rolled up to breath through and tried various ideas. The trouble with a fur ruff - say coyote - on a mummy bag is that it would interfere with closing it down. There are US military cotton face masks for extreme cold. My problems was breathing air at -40. A tent really helps a lot when its minus 40 because your breath and body can heat it and it can buffer the outside temperature.

I always imagined a fur tube about ten inches long and 3 inches across the inside with fur at least an inch long, the idea being that your expelled breath warms the fur and then the air you take in is warmed in turn. however a piece of rabbit fur over your face is a great help when its so cold that your lips freeze together.
Jim