Has your dermatologist done any other allergy testing, or is he focusing mainly on diet? I worked with a vet derm for a while and he did skin testing much like they do for humans. Allergy testing can also be done using a blood sample (if I remember correctly, these are sent to Michigan).
Good luck. It's so frustrating when they scratch like that. Tibby (my ruby) has no hair on her tail or her very bottom from scratching. She's on medicated baths right now. Previous vet didn't do much for her. Current vet picked her up and said "This dog has a yeast problem on her skin". It's not the definitive answer since she still does some scratching, but it has helped. De-sensitization injections did nothing for her.
RIP Wallis, John Robert, Tibby & Pip
Mom to JoJo, Roxie & Linguini (Cavaliers)
and my birds Moji (Senegal) and Walter (Blue Front Amazon)