Similarly, I picture drilling a hole in the center of the dent, installing a steel Nutsert, threading in a long bolt, and pulling out the dent. A hollow wooden cylinder could be shaped to follow the curve of the down tube, with a washer on top for the bolt head to pull against. (Wood, so that it wouldn't mar the finish, one hopes.)

The bike would be left with a Nutsert in a weird location, unless it was drilled out so that it could fall down into the bottom bracket for removal.

Of course, that would then leave a pretty good-sized hole in the down tube that would need to be filled, and might be a structural worry, so the cure might be worse than the disease.