Hi there, I realize this years after the question but I think I've found a way to assist. I'm currently using MWG 18.104.22.168 what I've done to implement what you're asking is to:
1. Have the Stream Detector running and finding streams and if it does then I create an event in that rule that sets a User-Defined property which tells whether or not a stream has been found.
2. That property is then used in a following rule as a criteria, along with URL.Host.BelongsToDomains(Youtube URLs), to find all streams that are NOT from YouTube
3. I'll then have the above rule block all those that aren't from YT and let the YT streams through.
It seems to work well so far but I am a novice at this stuff. I hope this helps anyone looking in the future.