I saw a post mentioning it on reddit, specifically referring to it being available for a few people via the YouTube app for Android, I checked and saw nothing on my phone.
Fired up YouTube in Chrome on my desktop and sure enough, there it was. Here's a few screenshots of what you see when you first visit the channel here. Assuming you have access that is.
I wanted to get as much of the info as possible on that last screenshot without cutting anything important off. At the top what's slightly cut off is "Why join the beta?", "With YouTube Music Key" and "Without YouTube Music Key".
Also here's the FAQs for it.
Also, if it's not on your phone but you have access online it will show up on your phone after you visit the site through your computer. Everything was normal on my phone a few minutes ago and then I went back in and noticed a change in when I swiped over with the intent to find the channel through the sidebar. Offline!
I guess I'll test it out throughout the next few days and then do a write-up on it next week.
So apparently it works with any video, since I clicked on that Marvel Civil War playlist and hit play on something on a whim and when I "exited" the app the video was still playing.
And naturally there's a new setting in the Settings menu related to the new functionality.