I have wrapped a single layer of painters tape over a painted tube, then used frame blocks lubed with marine grease to roll out a dent, with out marring the paint. I have significantly improved dents, but never completely removed one.

I also removed a dent in a chainstay near the bottom bracket. I inserted a socket close to the diameter of the tube into the stay, then used an allen wrench to load the end of the socket against the dent and pushed it out.

I worked at an aviation plant at one time, we had this tool which had hydraulic lines with different size balls on the end to feed into a tube and hydraulically expand to the dent from the inside.It worked, but we never implemented it due to quality concerns with cold working a tube. I tried to find details on it unsuccessfully.