As Matt said I would be careful using a dry shampoo on a hairpiece.

Dry shampoos are made to soak up oils from your scalp and hair, and since the hairpiece doesn't come in contact with scalp oils like growing hair does I would guess that is why Matt had such a problem when he tried it. The object is to keep the moisture in the hair to keep it soft and healthy looking. I tried a dry hair product powder one time and it didn't take long to realize it wasn't going to work for processed hair. Hairpiece hair is real hair but without the cuticle and without the ability to receive sebum from hair follicles it is a lot more fragile than the stuff that's growing out of your head.