This is MUCH easier to do at ADR times which are right at restaurant opening time like your is. We did the same at 'Ohana with an opening time ADR with no problems. For CP I would think there would be even less chance of being turned away because you'll hav already entered the park early for your ADR and once you're in there, they won't want to turn you away from the restaurant. The only thing I don't know is if they check the list to see if you have the right number of people when you're going through that early ADR turnstile at the park entrance. Anybody know?
That being said, if you're on the dining plan then I think I'd consider booking an extra ADR during the trip just to be sure that if you are turned away, you'd still have a place to use the credits that were supposed to be used at CP. I don't see you being turned away but I think I'd have that backup just in case (I always have backup ADRs in case we miss one or two for any reason; yes I do end up going to them and paying OOP but I would cancel if I decided not to).