Can Ford void a warranty? Not at all - other than for the exceptions listed in the warranty documents themselves such as racing, driving in a war zone, etc. Otherwise, the warranty cannot be voided (in its entirety).
The real question is can Ford refuse to pay for specific repairs under warranty due to owner modifications? Certainly under the law they can refuse to cover repairs of any part that has itself been modified or any part whose failure was caused by a modification. This only makes sense... why should Ford be responsible for fixing something that wasn't their fault?
The grey area is when Ford (or the dealer) claims that a failure was caused by a modification but the owner disagrees. Yes, Ford has lots of lawyers but that might not matter depending on the circumstances. Most claims would be handled in small claims court which has more relaxed standards. No matter how much Ford pays for lawyers, they won't convince a judge that a failure of the power windows was caused by an aftermarket exhaust. Even more closely related parts might still be a hard sell for cause and effect. As mentioned earlier, it would be difficult to prove that an engine failure was the result of exhaust modifications that were street legal (i.e. a cat-back system).
At some point even Ford is going to evaluate the cost of fighting (they have to pay all those lawyers) as well as the potential cost if they lose in court to determine if they will cover a repair under warranty. I think you'll find that they might initially refuse hoping the owner will give up but then give in if a real battle starts.
So, as Dirty Harry said, "Do you feel lucky?"
2011 Pony Pkg. convertible M6
Am WS7 (sold)