Only place I know of would be Bicycles of Ojai, in (you guessed it) Ojai.
Robert Coble is the owner, he did a lot of high-end steel stuff back in the day and now does some repair on the side. Not sure if he can do Ti (doubt it) but could probably do a stem repair. He can take a long time to get a project done, but he is a true genius in the literal sense and if you have a special stem you want to modify for long-term use he would be able to help you out I would think...although not sure why you would go through the hassle considering the time put in to modify a stem and repaint if necessary versus just buying a new one.
Anyways, good luck!
BTW for full disclosure I used to work at Bicycles of Ojai, but that really doesn't change my opinion too much on the matter.