Came looking for advice, left with the reassurance that comes with realizing that your plan isn't any jankier than anybody else's setup. I mount my light on the front of a porteur rack, so the cable has lots of area where it's just supported by air. Thinking that in those sections I might try enclosing the rear light and dynamo output wires together in heat-shrink with a 1/32" piece of TIG rod so that there's some support.