Back in the early '80s I made a bunch of touring bikes with wire guides all the way to the back of the bike. We snipped off one loop of stainless cable housing, the kind Campy and Suntour used to use for bar-end shifters. It looks better because it's wound from round wire, not flat like most brake housing. Hold that up to the belt sander in narrow-nose pliers, to grind it flat where you cut it, then hold the pliers in a vise, BO facing up. Melt a tiny drop of silver onto the loop, then hold the BO + silver against the frame/fork and heat until the silver flows onto the target. Takes about as long as it took to read that.

Only possible problem is we were using single wire, using the frame as the return path. I don't think B&M style dual wire would fit through that (try it?), maybe not even SON co-ax. They looked really cute though.