Rivendell and Tektro have developed a dual pivot brake with a 55 to 73mm reach. It is the model R556 for Tektro. The Rivendell version will have a slightly different quick release and brake pads. Rivendell also sells long reach dia compe centerpulls but your local bike shop might have some laying around.
Mafac Competitions have a very long reach and the rear brake has a slotted mount that extends the reach even longer. I ended up using two rears on my wife's mixte after I switched from 27" to 700c wheels.