It would not be creating any overhead. Just as the rocks do not add any - they are a part of the generated terrain. What you see as overload in Himalayas is actually because in mountains it has to create a lot more tiles because of the slopes, and since it doesn't compute occlusion (tiles that you actually can't see, on opposite sides or behind peaks, are still generated and rendered at the moment), it can run out of the memory allocated for terrain.
Snow would be generated as a part of the terrain, and the rocks and ground under it won't exist until you go through the terrain, making imprints or shoveling the snow away, which will cause re-generation of the affected terrain tiles. But that costs only when something changes, just like the craters, for example.
I understand then. Its better than i though at the begining.
With a coarser tiles for snow layer, maybe you can play with parameters. As we see in the video, in high altitude when slopes have less angle, then snow is accumulated and smoothed. As we see in the summits in 1:35 in the video.
If slopes have agressive angle,then ice is more flat and with more square forms.
Also sometimes snow is accumulated in high angle slopes, in the way we see in minute 2 in video. So you can play with posibilities, percentages, where sometimes (lower percentage) we have accumulation in high angle slopes (or have some square forms in low angle too).
Now when we see everest, the wall is almost naked of snow/ice. With this system can be covered at least half or 2/3, between holes, and generating more square form in the mountain, more agressive angles. Tipical in high altitude himalaya mountains.
I know also its a matter of retouch the parameters for the rocks in the high altitudes, i think they need more agressive angles too (more erosion there).
I am a high altitude mountains lover. Himalayas is a place that i like visit in outerra.
Its a nice challenge. Even for you (while try the task and see results), and for visit himalayas with great nice looking (now already have, dont want to imagine how can be with some improvements in that way, and it seems the posibilities for them are there).