Personally I like the idea of a heat shield, one that would provide an air space around the muffler that would position the cables far enough away to stop or reduce outer jacket damage.
Have you looking into better cables, ones with a spiral-wrapper metallic outer sheath? While not the least expensive route something custom-made might be the perfect answer.
I've used this outfit with good success: California Push-Pull Inc. Cable Specialists Based in Chico, Ca.
. They were willing to do a one-off project, cost was far less than expected and delivery time within 10 days. They seem very knowledgeable about push-pull cabling construction and function so can be helpful with final design. In fact if you had a damaged cable to send them that might be a great solution.
Just an idea.............