Whatever you do, be aware that if the main brake cable running from lever to yoke breaks, the brake return springs will draw the cantilever arms down and out, dragging the straddle cable onto the tire. If it gets snagged in those knobs, you might endo. Advise that you install something over the tire that will prevent a hay-wire cable from making contact with it. A fender is perfect. Failing that, run a long bolt through the fender-mounting hole in the fork, secured with a nut on each side of the fork, so as to protrude far enough to catch the cable. It doesn't have to be particularly stout -- M5 is fine. Since your current straddle cable is very long -- agree, it needs to be shorter, as above -- it could catch the tire well forward of the brakes and so your "shield" bolt needs to be quite long.