Normally the only way to fix "dents" is to put a lot of pressure on that area with a weighted item.
If you can find a way to balance the bag so you aren't pressing a heavy object against the entire thing that would work. Or one of those industrial wood shop adjustable clamps with wood blocks on either side with some good pressure would work.
I"d be afraid I'd have a "block shape" instead of a dent? But it sounds intriguing. hmmm