Great stuff!
I really like that you give user possibility to decide where he wants the dust to concentrate and where not. Having dust where it shouldn’t appear is as bad as not having dust at all. What bothers me though is uniform distribution of dust across the surface. Typically most of dust particles accumulate in corners and less on plain surfaces. Not sure about technical feasibility of fixing that in surface shader but maybe distance field nodes coming in 4.9 will let you improve that.
Cheers!