I'm pretty sure Airplay does exactly what you're talking about. If you play a video from YouTube or even streamed off your computer via airvideo, the apple tv connects to the server directly and the iPad or iPhone just acts as a remote. It doesn't use battery or network.
It's just seamless and you don't need to know how it works to use it.
I disagree. Not knowing how it works makes me not want to use it.
I don't know if you're right or not, but certainly YouTube and desktop iTunes is only a subset of AirPlay content. This doesn't really scale without providing a spec for content to follow.
All iOS apps must use HLS for videos longer than 10 minutes and Apple verifies compliance during the app store review process. As mentioned, with AirPlay all HLS media files are downloaded directly by the AppleTV, not piped through your device. I can tell you from experience it works very well.
You can put the iPad to sleep and it will continue to play the video on the AppleTV, but my guess is that if you turn it completely off or move it outside of wifi range you'll lose the control connection so the video will stop playing.
It's just seamless and you don't need to know how it works to use it.