For what it's worth, I've been looking for a dust shield that's actually a series of fan type blades that will promote airflow through the rim and hopefully reduce the dust build up.
Haven't found them though...

NOTE:
This was typed using fingers permanently stained with the black crud that cleaning with simple green and a piece of open celled foam creates.
Cleans the rims easily and very well, but I should have worn gloves...

I too believe that the best stopping power is the preferred condition, regardless of the dust build up.