Not sure if this will help you, however I can do this quite easily using a Plex client on the iPad, linked into my Plex Media Server. I have the PMS on a NAS, however you can just as easily run it from another PC or Mac. You can select a show on Plex (on the iPad), select the ATV (using AirPlay), and then minimise Plex and continue to work on the iPad. I also have an Apple TV (Gen 3) which is awaiting jailbreak / new Plex client, and this is how we us it to watch media. I have an older Gen 2 ATV that has been jailbroken and has the actual Plex client installed. I was very impressed by how well the iPad / Plex / Airply combo worked, even with very poor WiFi signal strength.
Anyway, just one way to be able to watch a movie (albeit with content from a media server) and continue to use the iPad - I acknowledge that this may not be 100% the solution sought, but just an idea.