Probably won't find a dermatologist that "just" does horses but you may get a veterinary dermatologist who has an interest in horses even though their major focus is small animal. In your area I would put in a call to Marion Du Pont Scott Equine Hospital and see who they recommend.
You might try a trip to Morven Park. They can biopsy the rash and send it off to a pathologist. Also, ask them to photograph the rash. I think they have a dermatologist at the Blacksburg campus who can review the pictures if you ask them to forward them to the dermatologist.
Yeah, I haven't been able to find any on staff around here. Equine dermatology seems to be a very, very rare specialty. Only found a couple on the Internet, very far away. I'll try calling around, but I'd really like a specialist to see this condition. I've already been through several vets that were SURE they knew what it was and how to treat it.
Tranquility Farm - Proud breeder of Born in the USA Sport Horses, and Cob-sized Warmbloods
Now apparently completely invisible!