You are correct that mastocytosis, in which the skin is infiltrated with many mast cells that release histamine and other chemicals, is rare. No doctor or clinic's name in particular springs to mind. Certainly the folks associated with Mayo would know what to try. I suggest you get in touch with patient support groups for this condition for tips on treatments people have tried and physicians they have consulted. One place to start is the Mastocytosis Society at http://www.mastocytosis.com/, but I'm sure a web search will turn up others.
Sorry I can't be more specific. I've never seen a case myself.
I have commented below under another person's post on Urticaria Pigmentosa. Isn't that another name for cutaneous mastocytosis?
(as nearly as I can tell from mastocytosis.com, it is)