Having a stove that doesn't work when it's -37C is the least safe option of all. If you really want to use an LPG stove in super cold weather, there are a couple other options.
1. Get an LPG stove (or modify one) to take in liquid fuel instead of vapour. The MSR wind pro stove lets you do this by turning the canister upside-down.
2. Make a canister heater by wrapping some copper wire around the canister and sticking the end of the wire into the flame. This method is also "Not recommended" by the manufacturers of LPG fuel canisters, but at least you are only increasing your risk while cooking, not all the time. It would be wise to monitor the temperature of the LPG canister so you don't overdo it.
As for the safety of running propane in a canister designed for butane, it's really a matter of the storage parameters: Maximum storage temperature (which governs the pressure), and acceptable probability of a canister failure. Since we don't know enough about the reliability of the canisters that are pressurized beyond their specification I won't go there, but we can analyze the temperature and pressure. Primus specifies a maximum temperature for their powergas canisters of +50C (125F) http://www.google.ca/url?sa=t&source...9x2tg5tVcNLD9Q
I'll assume that other canisters are rated to a similar temperature. Here's a good reference chart for the vapour pressure of propane and propane / butane mixes: http://www.engineeringtoolbox.com/pr...ix-d_1043.html
If this table is correct, then 30% propane / 70% butane at 110F is 93psi and is "safe". Pure propane at 60F is also 93psi. So you just have to keep the propane canister below 60F (about 15C) to be "safe". That should be no problem in Alberta in the winter. Probably if the air temperature actually hits 15C you should drop everything and run before you get smoked by a huge avalanche. It's more difficult in Arizona in the summer. Just make sure your on top of it and empty those canisters before spring sets in.