Disney has, in the past, barred adults from entering the parks dressed as Disney characters. The most recent one I recall is a young woman who was barred from entering DAK dressed in a very elaborate Tinkerbell costume with her boyfriend on his birthday. The company line was they did not want guests confusing her with CMs in costume. I've also seen adult guests in extremely well done, highly detailed character costumes at
MNSSHP. There was a guy dressed in an off the hook Mad Hatter costume at one of the MNSSHPs. It looked straight out of the movie. So it's possible that they are a little more relaxed at Halloween. I've never heard anything like that on
DCL and I've seen more than a few Captain Jack Sparrow lookalikes (some better than the actual costumed CM) on pirate night. I'd place a call or send an e-mail to DCL just to be safe.