I've seen some manufactures of speakers use dust caps that are made of a mesh that would block dust but still allow air to flow through them, like a furnace filter.

Probably would work well for cooling the voice coil, but in a sealed box, isn't that asking for trouble because of the potential extreme pressure in the box?

 

which sub has that just out of curiosity?

 

I forget who made them, but I remember once taking a look at a sub in a store in downtown Toronto, and lifted it up to look in the pole vent, and remember being able to see light rays through the dust cap, but there weren't any actual visual holes.