That'd be easy enough even before the release of the SDK - open the video on the Chromecast, audio on the sender app, use the websocket connection between them

That'd be easy enough even before the release of the SDK - open the video on the Chromecast, audio on the sender app, use the websocket connection between them to keep them in sync, Bob's your uncle. Making it so that you only downloaded the video on the Chromecast and the audio on the sender app would be a little more difficult, but not insurmountable.