I've read recently (looking for post) that DCA was kept open for an additional hour recently to handle the MK overflow. Also last year when the MK was setting record headcount numbers, the officially extended DCA from 6 to 8pm for a couple weeks. I think they also added a late Fantasmic show on some of those days.
Even over this holiday season, they extended hours a few times. I'm assuming if crowds are large enough, they will continue to try to accomodate the overflow.
There will be a message on the PA that says so, but often this is missed because you are inside an attraction etc. Best bet is that you don't leave until you hear the announcement that the park is closed &/or a CM tells you to leave
Our Dreams Unlimited Travel Agents will assist you in booking the perfect Disney getaway, all at no extra cost to you. Get the most out of your vacation by letting us assist you with dining and park reservations, provide expert advice, answer any questions, and continuously search for discounts to ensure you get the best deal possible.