When there were too many Vintage dailies, there was a concerted effort on the part of the Vintage playerbase to select a specific daily and all show up for that one. If we can get that same kind of effort going, I think leagues should be no problem. The "any time" aspect of it could be both a help and a hindrance.
I think it'd be really neat to get something set up where a bunch of people stream league matches at once and viewers jump between based on who's currently playing or who's got an interesting match. I'm picturing something like a voice chat that only streamers can talk in but everyone can hear so I can say "oh snap, this guy is playing Dragon, ya'll get in here!" or maybe a bot that sits in all the Twitch chats and advises when new matches are occurring on other channels.