If you want to get fancy you can obtain threaded ends for carbon and fiberglass rods. Central Hobbies is a good source for those. Although I agree with w8ye about the harmonics, it's very difficult for that to be transmitted if the is a non conductive break in both ends of the rod. Simply useing a metal rod is bad because they are extremely flexible and vibrate quite badly with engine rpm. Using straight threaded rod is absolutely terrible since it is brittle due to threading and will snap quite easily when subjected to engine vibration.

For throttle linkage, I prefer the most rigid material I can find to avoid "thermal creep". Many of the flexible rods change dimensions with an incease in temperatures, changing throttle trims and end points. If you have to route a throttle control around anything you're stuck with a flexible arrangement. With large gassers that is rarely a requirement.