"Would not assume?" Sure, they *might* fight it. And have it public knowledge that a kid died in their car seat, for whatever reason. And then they would have to try (in vain) to produce any real world evidence that could even possibly back up their assertion that thin strap covers, or an after market cover, or a head hugger, or whatever, caused the failure of the restraint. Which would then be them admitting that their seats are so abysmal at performing under real life circumstances that they can't handle a snuzzler (perhaps one of the worst offenders, both in quantity and quality). But then all you need is an attorney who has at least read one book, because any defense attorney worth his weight in fertilizer would win that case, hands down, open and shut, period the end. If it truly was seat failure, and not blatant misuse completely independent of seat failure, then their is no "assuming" liability, it's liability period. Your question makes the naive assumption that they have to claim guilt in order to be held accountable, which is far and away not the truth.
If a harness rips out of a seat, I don't care how many strap covers you have on it, it's the manufacturer's fault, no questions asked. No way would they ever fight you on it, because then it becomes public knowledge that they a) produced a a faulty seat causing and/or contributing to a child's death, and b) tried to force the blame on something/someone else with a theory that a four year old can see through. If the shell on the seat cracked, or the anchor belt snapped, or the harness released, or any of the other lovely and potentially fatal failures occured, it's not because of your Prince Lionheart car seat mat, or your PiddlePad, or your Snuzzler, or you strap covers, or even those stupid dangly bug things hanging from the handles of baby baskets. There is basic engineering quality that is to be expected, and NONE of these products can possibly impact the integrity of the seat's construction.
Now, the topic of misuse comes into play. A Snuzzler often keeps parents from tightening the harness enough, but parents don't tighten the harness enough anyway. You CAN tighten it plenty, thereby eliminating the possibility of ejection independent of basic seat failure. It's not a down comforter, folks. There is not gonna be THAT much compression (and in a RF seat, mind you, where the harness does pretty much nothing) that the kid is gonna fly out. Neither with strap covers, and a seat saver is (last time I checked) not made by 2x4s or even industrial grade steel. The rubber ones are a bit ridiculous, but they're still not death in a box as many would have one assume.
The only after market product that I really truly see a valid claim of voiding liability on is the Mighty-Tite, and even then it would not void Graco's liability, but Ford's. Graco might get in on the action, and have a valid argument, but that is moot anyway because SKJP would be going down for that debacle.
For the record, there is a towel under my kid's car seat (I think... don't even remember actually), and I would absolutely use the harness covers from his Regent on the baby's SS1 if I felt the need, and if for some reason my infant is having major head slouch and the SS1 insert doesn't cut it, I have no qualms buying an after market head hugger. I would completely and absolutely buy an after market all cotton cover for his car seat if his skin ever started acting up again, and wouldn't really worry in the least if it was made to the same pattern by a competent seamstress. And if my kid screamed non stop... no, I still wouldn't buy the dumb dangly bugs, because they just annoy me. And don't get me started on the three pound hard plastic Baby Einstein aquarium dumb thing secured by two tiny strips of velcro SUSPENDED OVER YOUR BABY'S HEAD (!!!), that's just plain dumb.
I would not use a mighty-tite. Or a super thick rubber tire mat. Other than that, I am confident that there is nothing any after market item is going to do to harm my seat or endanger my child, so long as it is used for its intended use and the seat is still used correctly.