I don't know if anyone has enough magic to braze a new guide without damaging the paint. You could remove the broken one with a file or dremal tool (carefully). I'm afraid your options are to glue another one on or opt for a clamp on guide. This is not a highly stressed part so JB weld just may work fine.

Nova Cycle Supply has most all braze-ons.

http://www.cycle-frames.com/bicycle-...GUIDES-c-1029/

Good luck