Neat trick!

 

I'm no expert by any stretch but the problem you are having is that when you weld and it cools the metal near the weld shrinks. so when it shrinks a little over a few inches it will pull the crown out of the panel. so to fix it you need to stretch the metal where it has shrunk. you can do this with a hammer and dolly but i don't think you could get a dolly behind that metal.
also if you heat the metal up and cool it you'll only shrink the metal farther making a bigger dent.
Here is a good site with a bunch of metal bumping geniuses that you could ask there.
http://www.metalmeet.com/forum/index.php

 

That's pretty cool. No pun intended. I may have to try it.

 

the dry ice trick, i wonder if that works on heavier gauge metals. im sure the steel in your tailgate is a bit thicker than on the mazda in the video. i could be wrong though....