The rules state no full face mask. So, if you decide to take your chances..be prepared if they ask you to take it off. I think all costumes are at their discretion(meaning, they can choose whether to let you in or not). Also, if you look "too much like the real thing", take too many pictures with guests, or cause any kind of disturbance...they may ask you to leave. I've heard of them doing this and one year a CM warned my husband "to be very careful while taking pictures with guests cause if security sees you, you may be asked to leave". He was dressed as a Davy Jones' cursed pirate and people kept begging him for pictures.
So, use caution and my tip...better to play it safe than sorry. I've heard stories of people who spent tons on their costumes and then were asked to removed parts...which made the costume look silly.