Join Date: Feb 2013
Location: Near St. Louis, Missouri
Bikes: Giant Defy Advanced, Windsor Tourist
You might even be able to get rid of the dent with a long metal seat post or tight fitting metal tube. Bevel the end of the tube a little and slide it down the seat tube. When you hit the dent, tap the tube or seat post gently to see if the dent comes out.
If you were really worried about the dent, you could leave a piece of pipe inside the seat tube as a doubler. Probably not needed though.