After stopping the 10th or so new render, what I can say is that this thing is highly sensitive to even minor changes in the parameters. Tune something up from 1.0 to 1.2 - boom, everything goes grey. Tune a density down from 0.1 to 0.05 and you don't get half as many snow flakes, you get almost none at all.
I have a promising image coming along nicely right now, but there are so many parameter changes I'd love to try, and no time to do it. Not enough CPU power, in fact.
This is a very interesting approach, though. I think I'll be trying a modification for rain (stretch in Y direction, more grey than white) next, as soon as this one is done...