I dont think the fishing rod can't take the loads, but I think it will be too flexible. You want something fairly stiff and the best way to increase stiffness is to increase the diameter of the tube. It is nice to have something fairly substantial to hold in your hand as well, but that can always be fixed. You will need to watch the weight on the end of the tiller when you flip it around behind the mainsheet when tacking/gybing.
We will probably make some carbon tubes for our boats. We need tubes both for the tiller and the tillerbar. That way we can always make some extras as replacements "when" (knock on wood) we break them.