Ouch. Rereading my source above (Gladman 2006), they say hyposensitization (desensitization, if I understand correctly) generally doesn't work, but ...

"Lasting natural hyposensitization has been reported, however, among Japanese woodworkers who use lacquer derived from Toxicodendron verniciflua, the Japanese lacquer tree. In a 1991 survey, 81% of craftsmen developed dermatitis from the lacquer but 83% of these reactions resolved with continued exposure."