Eventually the Chromecast Beta app might be enough. Google has been making changes to the way chromecast works on android tv (I just got the option to add the shield back into speaker groups today!!) This all may work by the time Google releases their new android TV/ Chromecast device.
From the little testing I did It looks like you can get the status (idle,play,pause,buffering) when you are casting to the shield, and from certain apps like youtube.
Unfortunately at this time it doesn't update the status for Prime Video, Youtube TV, and Kodi.
Prime Video App on the shield:
When casting Youtube TV from my phone.
