I think I'm more concerned about myself than I am my horse during cold weather :) I'm not into frost bite, but I do ride when its in the 30's 40's F. It doesn't get too cold here, we are pretty far south near Oklahoma. I do consider the footing when riding. If the ground is very frozen, it is harder on the horses feet. Ice is a no no for me too as well as wet sticky snow. I guess as long as your comfortable and the footing isn't too bad, go for the frozen nose
I use to live in montreal but had my horse out near the ont border, we went trailing all winter and rode in the indorr arena when weather was to bad, but I never rode out side if it was over -10 c and about -20 if I rode in the arena, the horse was more bundled up that I was walking back across the street to our barn.