Okay looking forward to seeing it BTW Just for fun I was messing around with this today and came up with a mixed material, it doesn't have the ease of use and control like with layers, but it's workable.  I based the idea upon the flame mats, using a Step (smooth) function as the alpha control to get the transition between the mats. I found i had to create a mixed mat with the rock and grass first and then mix that with the snow, here's a some shots of the function settings.

This function would be applied to the rock and the snow mats.