The legal columnist in the AOPA journal cited and reported on a case where an aviation related waiver was upheld. I believe it revolved around formation flying. Some of the elements that made it stick involved the fact that the pilot who signed the waiver (holding the other pilots and the aerobatic association harmless) had the opportunity to sign this in the comfort of his home and avail himself of legal advice.
I don't mean to suggest that it's not possible to have a liability waiver that will protect you from liability - it certainly is.
The problem is, as pointed out above, there are a whole bunch of potential problems in getting one - was it "fairly obtained," or "did the other party know what it was doing when he/she signed it," etc. Those are some of the "procedural niceties," for lack of a better term, involved in obtaining a valid signature/agreement on/to a waiver.
And, even assuming that you do get a valid agreement to a waiver, you are not allowed to disclaim your duties in certain settings (which is what a waiver is - it allows you to operate without regard to the consequences of what you're doing) - namely, you cannot contractually disclaim a duty that is imposed by law. And,
in the aviation setting, a lot of duties are imposed by law.
Oh, and on top of all of that, there are also certain
kinds of negligence that cannot be disclaimed. It really depends on where you are, but some states don't allow you to disclaim really egregious negligence (willful/wanton, gross, etc.). Say, for instance, you did something
really dumb as a commercial operator, like not check the tanks for how much gas was actually in them, or not doing a walk-around and missing a brake hose that was disconnected, or flying a grossly overloaded plane. Even if you've got a waiver, it might not cover things like that.
To illustrate the rule-driven nature, though - setting up the NPS claim - wouldn't those three things I listed in the previous paragraph involve some kind of violation of a rule?
So, you're better off paying $X for a really good insurance policy, rather than relying on some paragraph in a contract. While the latter
might work, it's so full of holes that I tell anyone who asks that they're taking a risk to rely solely on it.