I was smoking a cigarette (terribly bad habit, quit if you can!), and it occurred to me. The heightmap node (not the operators) replaces the terrain in it's area with whatever it's generating. This actually means if you mask it with the same relative terrain, it will form a complete terrain. This means you can use the heightmap generate to smooth areas where your snow is! It's actually very straight forward and I tested it with roughing up the original terrain before going into the heightmap to make sure it was doing what I wanted.
The resolution in this test for the heightmap is 20000x20000 at 2000x2000 points, which is too low as you can tell by the pixelated designs. It does unfortunately take a LOT longer to generate the terrain if you're applying secondary PF displacement to a Classic Erosion terrain, however is fairly quick on the Classic Terrain Heightmap itself, which is to be expected.
The benefit of this method is the heightmap doesn't actually alter the computed terrain, so the same distribution shader doing the masking, can also mask the snow texture accurately. The snow surface layer has smoothing on, as in the fuzzy zones of the heightmap, the original add-on PF details will show through, and the fuzzy of the surface layer with smoothing can help a bit. You could even adjust the distribution going into the surface layer.
Update: Turns out it's not as straight forward as I thought. In order for this to work the original terrain also needs to be a heightmap. For some reason if the underlying terrain is just a PF mountain (like the default scene), the masking doesn't work, or it doesn't smooth correctly, something. But if you also run the base terrain through a heightmap generate it works. I don't know what that's about. Probably why it worked first time around on a eroded terrain cause it is a heightmap.