Dan, I'm not trying to be difficult or a legal-beagle, but if my lower caes gears got torn up due to a warranty defect having nothing to do with a foil on the cavitation plate, there is no way I wouldn't challenge the manufacturer if they tried to deny a warranty claim, regardless of the warranty language.
Funny, over on the Sport Trac site, there is a thread going right now about a guy who had a wheel fall off his Sport Trac yesterday. Now, he has a 4" lift kit installed, heavily modified suspension, and he cut the A-arms himself for the modifications. Should Ford deny his warranty claim? Absolutely! It's more than likely a self-induced problem.
Common sense should prevail in these situations: If there is cause and effect from the aftermarket part and a failure, an OEM warranty should not cover it. If there is no relationship between an aftermarket part and a warranty failure, then the manufacturer should not try to hide behind the warranty. It's disrepectful to the customer to do so.
Sadly, there is too often dishonesty from both sides in warranty issues: Customer often says "I didn't do anything" when in fact they did do something. Manufacturer often says "we're not responsible", when if fact they really are.