As part of the recent security changes (see this thread) there was mention that no more costumes would be allowed for adults. Nothing was mentioned at the time if this would apply to special events as well. The following just came out in regards to the upcoming runDisney events. Per the part I bolded in purple at the end, this seems to suggest that adult costumes are still off the table completely inside the parks. http://www.wdwmagic.com/events/walt-disney-world-marathon/news/05jan2016-disney-clarifies-costume-policy-ahead-of-this-weekends-walt-disney-world-marathon.htm