I solved that problem by replacing the "upside down" mixte rear brake with a standard Shimano 600 sidepull and routing the cable up the seat tube and then down to the brake. Then I learned that nobody makes a 1 1/8" cable clamp. I ended up using a Sturmey Archer fulcrum clip clamp with a short piece of vacume hose th increase the diameter of the cable so the clamp would grip it tightly. There's a lot of friction in the system due to all the bends, but it works fine with a sprung brake lever. I think you can see what I mean in this picture. I wasn't using the SA clamp at that time. It was one that I made from a braze on FD adapter that didn't work out. I realize that you don't want to make such extensive modifications, but maybe somebody else can use the information. Besides, I needed to kill some time while the artichokes are steaming.