The Reactor melts snow like a bomb - until the canister freezes. IF you can reliably keep the canister above freezing point then yes, it is fine. But doing that safely requires a bit of skill and knowledge. You may have that, but I cannot assume that.
Now, Jim's question about operation at 3,000 m or when the pressure has dropped to 70% of atmospheric. Good one.
This is a plot of the pressure of various gases and canisters down to -35 C. Note especially the horizontal black dashed line. That is 1 atm or 14 psi. Below that is the 10 psi line - this is about 0.7 atm.
The BP for n-butane drops to about -10 C and for iso-butane to about -22 C. Hum - getting interesting.
The static pressure for something as low-performance as a Coleman canister drops to about -33 C (the curve is under the ones for MSR and Snow Peak). To be sure, if you are evaporating gas off the canister it will be cooling, but if you can keep the canister above -10 C the butane will still be boiling away.
So ... yes, at high altitude things actually get easier!
If you want to melt snow with a Reactor at high altitude, sit the canister in a bowl of cold but not frozen water. It will work. (Actually, that works at sea level too.)