I have no knowledge of whether soy sauce permanently stains or not, but you can sure tell its aroma. I'm surprised you could sit in soy sauce and only think your pants were "wet". I agree that negotiating in the moment is always best - asking up front for cleaning would be the first course of action. Let the hotel take care of it with their in-house cleaning services. Then they could see themselves that the stain was permanent, and I then think your starting point for negotiating a settlement would have been much stronger.
The problem with Marriott is that I've been promised points on several occasions (nothing as serious as this) and even after following up they have never appeared. I will also be waiting to hear the outcome - not just the promise of points, but whether you actually get them.