Based on the information in your post I believe the best thing to do in your particular case is to replace your seat cushion and let it go. Judging from you and your son's earlier posts, you've put a lot of time, blood sweat and tears into your hunting property. As has been said, you shouldn't involve the land owner, and as you said, if you do, you might lose access to the property.
It's human nature to want to find out who it was and thump some heads, but again, this would probably get back to the land owner and once again, you might lose access to the property.
It's just a seat cushion - easily and not too expensively replaced. If I were in your position, I would, as suggested before, "take the high road" and just let it go.
Besides, there's always a chance that there are a bunch of squirrel nests in the trees around that stand that are now lined with chewed up foam from your cushion. They were probably all up there watching from their leafy little houses and laughing about it.