I have seen people use a paint brush to brush down velvet-covered
display stands/trays. It looked like a good idea to me if all you
want to do is brush off the daily dust/pollen/pollution you encounter
from outside shows. I don’t think you should get velvet wet, though.
Maybe a damp sponge would be ok.
Also, one painter/artist told me that after an outside show he goes
to the gas station and uses their air pump to hose off his paintings.
That might work on velvet.
I’ve also tried using my vacuum cleaner, but probably the easiest is
to just use masking tape rolled around the palm of your hand.