agreed. The insulative properties would keep the heat out and away from the water...to the point where you'd likely damage the Thermos.

What you might be able to do is half fill it with hot water and fill it the rest of the way with snow or ice and let the temperature equalize. Very little heat would be wasted while this equilibrium took place...unlike boiling where as you melt snow and ice, the pot and the water within give off tons of heat into the atmosphere the whole time.