I have the 4K PVR. I just noticed today, from my Laptop, while viewing Youtube, the CAST option had OPTIK as an option. Lo and behold, I could cast directly from Youtube to my Optik box. It actually launch a Youtube app. Exiting the app, and pulling up the Optik app list, YT wasn't there.
Launching it again by casting to it, I can navigate within the app and search other videos.
Did a test from my Android phones Youtube app... and OPTIK is there to cast to as well.
I'm guessing since it's not in the app list, it's still under beta or testing
The app still needs some work. It doesn't seem to have as high as video quality as casting to ChromeCast - and it keeps freezing after 5 or 10 minutes. Sometimes it restarts and sometimes I had to exit back to the home screen or even exit out of the app and restart. The nice thing about having a YouTube app is that you don;t have to tie up your computer or mobile device to watch or cast videos. I'm busy watching the Euro 2017 Women's soccer championships - and have given up on the app because of the freezing but hopefully that will be fixed soon.