The process is to fill the pipe with water just past the level of the dent, and place it in the freezer. You don't need to cap either end (supposedly) as that may in fact cause the pipe to burst. Luckily my header pipe is small, and fit on the beer glass shelf:
I'll see what effect the frozen water has on the dent in the morning. It may be something I need to try multiple times.
Well, my result came out slightly different. The ice did in fact remove some of the dent, but in the process split the pipe open along an apparent weak spot/crack. I would say about 80% of the dent has been removed, and I can easily hammer the crack closed and stitch weld it.