That's a really nice work indeed. If you can work in external modeling software, you can take a tree and model some blobs of snow on the larger branches, as an extra object, or integrated with the tree object. When populating, the blobs go with it of course if integrated, else distribute with exactly the same parameters (seed!). I tried that and it works well. Then of course use a 'whitener/snow shadermix' for snow on the actual branches and needles too.
Unfortunately you can't populate random blobs of snow sitting randomly on a pop of trees (yet).
