CyberChrist
You caught my attention
Sadly not. Placing a Volcano in the middle of Snow-Capped Mountains does not produce a Snow Volcano and neither does placing the Volcano first and then surrounding it with Snow-Capped Mountains.I think they have to be in the midst of snow mountains.I can't recall off the top of my head how/if snow volcanoes work?
In fact, I have never been able to place a Volcano making use of the "Volcanos-snow.pcx" graphics, but just in case anyone knows how to accomplish this ... then please share.

They probably work exactly like Snow-Capped Mountains(controlled by a variation bit?), but Firaxis just forgot to include them in the editor on the list over placeable terrain.
Too bad they don't work like Balthasar thought. Good thought on the variation bit. I took it for a spin in the hex editor just now, trying to get the game to display a snow-covered volcano.
I suppose this includes LM-flag combinations as well?
Hard-coded afaik.