Palm strike to the face or nuts? Remind me to wear a thai cup and a motorcycle helmet when rolling with you.
j/k, I think different people have different feelings on it. I'd just ask, mid roll even. Some people use the palm to cover the nose and mouth while having the opponent's back. Legal? Yes. And I wont' bitch if someone does it to me, but I've heard people saying it's disrespectful. I think just asking is the best way to go.
Oh another one is the Douchebag arm bar defense break. It works, hands down, although I've even heard a brown belt complain about it saying, "if someone does that to me, they better expect my toes in their mouth when I get the arm bar on them." Just ask.