From their point of view, you're allowed to saw off anything you like? And, I personally wouldn't be worried about "voiding the warranty", because A: you don't need it, because they don't warp and B: if I sawed off a little bit from the headstock and then, I don't know, the trussrod fell out or something, I could make a very persuasive point that my little chunk-removal wasn't related to the trussrod. People do modify these parts all the time.