There is a tool made by Bicycle Research that clamps on the tube and is worked around to remove the dent. The inside diameter of the clamp is the same as the outside diameter of the tube.
I've got one of these tools. It is sized for an unpainted tube, so even a thin coating of paint will cause scraping of the paint and uneven "popping" of the dent. It requires a very strong vise and careful loosening, tightening, rotating, to do the job right. Even after all that, it doesn't completely remove the dent. There is usually a small crease that is visible. Lou Deeter, Orlando FL