I would say NO because they do not allow costumes on adults in the parks otherwise. They only allow adults in costume during the Halloween parties. That means only at Disneyland starting at 3 0r 4 depending on your party day. The rule would still apply at DCA.
The Halloween parties are coming up. That is the only time adults can ware costumes to the park.
"The Park" being DL during the party, not DCA... just wanted to let everyone know I called twice about this and they said they make no exceptions at DCA. Altho, sometimes they have been wrong, but it is pretty clear on the website, so I am sure they are not wrong in giving their answers to this topic. If you want to risk it, I would say that your costume should be able to be removed and stored.
My friend last year wore cover-alls this was his outfit for the party + mask last year. He simply put the mask in a locker til the party started and had no issues hopping all day between parks. So if your costume has "normal clothes", you should be able to take off the different parts that make it a full costume.
I also know of someone who was going as a witch, her black dress was clearly designed to be as a H'ween costume, but without makeup, she too could park hop until the party started. She went to her car to apply the make up and hat and broom-like prop.
The more detailed your costume is, the less chance you can attend the day hours before the party.
"Ahh-chooo!" ~ Walt Disney "Bless you." ~ My Grandfather (Disneyland, circa 1957)
Last year, while in line for Tower of Terror, there was a family in line, dressed up like the Scooby Doo gang, saw them later in the park and they were still in costume. This was an hour to half an hour prior to the beginning of the party at Disneyland.
The policies on costumes changed when the format of the party changed from private only to the mix-in start. If I remember correctaly, costumes are allowed all day in both parks on party days. Other than party days, only children up to 12 (higher than the usual cut off of 9) can enter the parks in costume.
They has been no announcement if this year will be the same policy.
And I also notice that what is defined as a costume gets a little relaxed during the Halloween time promotion. So more extreme styles of fashion that normally would not be allowed are allowed.
I should probably mention that some costumes ( ones that involve proton packs ) aren't allowed. Me and my group wore ours in 2008 in DCA with no problems but I've talked to others in the last few years who've gone and been turned away by security for safety reasons. The moral of the story is keep your costume simple and capable of mixing in with a crowd, no Robocop armor. On the flip side my friend's Rocketeer costume ( with jetpack and helmet with big fin on the top ) seems to make it pass this inspection every year. Hmmm,... Either way, buyer beware, no complicated costumes or you'll be turned away.