I recently wrestled with this too - fitting a Pizza Rack to Surly Cross Check that had Tektro canti brakes - but found the cable guide interference was only half the issue. The other problem was getting the rack close enough to the fork without it pressing on the brake calipers. May not be an issue with different canti models idk.
Anyway, the solution to both issues for me was to buy some mini-Vs instead (£12 on Wiggle). They don't use the midfork bolt and they are nice and slim so no interference.