Update: the functionality used by the application was reverse engineered and Google stated that it may change. Google has released a statement which acknowledges that playing local content will come back to ChromeCast once the API has stabilised. Storm in a teacup, apparently.
Heads up. Google's latest Chromecast update intentionally breaks AllCast. They disabled 'video_playback' support from the ChromeCast application.
Given that this is the second time they've purposefully removed/disabled the ability to play media from external sources, it confirms some of my suspicions that I have had about the Chromecast developer program: the policy seems to be a heavy handed approach, where only approved content will be played through the device.
A company intentionally disabling cool stuff? Surely you jest.