Road frames with braze ons for the FD aren't made with the contemplation of anything smaller than a 50 tooth big ring. Perhaps an extender could be fashioned by a machine shop. Bolt the extender on the brazed on fixture and then the FD onto the lower portion of the extender. Basically a slotted piece of aluminum curved along the long axis to match the curve of the braze on type derailleur mounting stud. All you would then need would a SS steel machine screw and nut to attach the extender to the brazed on fixture.