Helicopter tape is the name used by the mechanics at airports. If you are near a small airport, you can get it there. It comes in rolled sheets. Its very thick and I understand the auto track race mechanics use it to protect their expensive car paint job from chipping from road grime.

This tape won't yellow over time and when applied, it can be removed without ruining the paint job. But maybe an added thing you can do is to do what Felt does. They put a rubber sleeve on the cable right where it hits the head tube. Supposedly the rubber sleeve won't mar the paint.