Ultimate does do custom work.... but I don't think there are returns on custom stuff like that (something to consider).
As for re-doing the Corbin Stinger.... I would have to recommend that you don't do that, sell it and get a stock for someone to rework if need be. There is a guy on the VTX board that reworks seats (any seat I think)
http://www.meancitycycles.com/..... but I would recommend you buy a stock seat pan for him to use and sell the Corbin to help recoup some costs.