You have come to the right place for help, but frankly I can not remember reading where a pitcher can not wear sunglasses. There are regulations for color of gloves and arm bands but nothing on sunglasses that I could find.
If you keep them to all black and not with a reflective coating I could not see how this would give anyone a case to not allow him to wear them.
Piaump or someone else will have some advice that is relevant.