my only thoughts on this besides postwork, would be that while dry hair tends to fall uniformly and evenly, wet hair tends to clump together in strands. with that in mind I would say you might try modifying the trans map to match that effect. if you then also morphed/scaled the hair to be tighter to the head,implying weight it might add to the illusion. last thought would be, depending on the type of hair, to add an oversized  prop version of the hair over the top of the conforming one, and make a water drop transparency map for it, although that might just be the mad scientist in me.