Yet another option: If you can find a steel pipe or rod of a diameter close to the inside diameter of the seat tube, you could pre-lube it and gently tap it down to where the dent is. Carefully push out the dent and check for cracks. If the tube is slightly bent, this method could even straighten it. You have little to lose. Aluminum is more malleable than some people think.

I actually own a large steel drift that would work. No idea where it came from, but there must be others online in the $10-20 range. You would want to be sure you could draw it out again, so pre-greasing is important.