Thanks to every one who responded to my question. Several people suggested replacing the limit screw with a longer one. I've tried that with two modern derailleurs, a long cage mid-range Campy (I forget which model.) and an SRAM X.0. Both had the problem that with a five speed cluster the parallelogram doesn't swing enough for the stop to ever touch the limit screw. A longer screw doesn't help in this case.
The strange thing I discovered with the X.0 was that an old Campy bar- end shifter has exactly the right amount of cable travel to work with a five speed New Winner and no low gear stop is needed. (The shift lever is the stop.) The combination is the best shifting I've ever experienced. It's much better under load than a Duopar, looks cooler than a first generation Rally, and can be taken apart like an Allvit or Mavic. Unfortunately it needs a huge amount of cable pull. I'm tempted to try it out with a old Simplex tennis racket lever with a big barrel.
As for my son's Rochet, at this point I'm probably going to use a Nuovo Gran Sport or Sun Tour out of the junk box.