If the answer is proprietary red tape from Google, is there a front runner for a casting protocol or dongle we can use?
Basically yes. The Chromecast API is not open. There are reverse engineering efforts and I think some basic functionality (like streaming a video) is open. But in general it isn't.
I remember at some point Firefox on Android could cast videos from web pages because IIRC it uses the Android-built-in Chromecast SDK. But this functionality seems to have been lost.