Preroll-disable functionality already exists, just have hypetrains add 5 minutes to that when they start, if the counter is at zero. Add more time if level-ups extend the hypetrain duration.

If a hypetrain is going, something hype is happening, right? Who wants to come into a channel during a HYPE moment, only to get slapped in the face with a preroll instead of joining in with everyone immediately?

The preroll revenue loss would be minimal, would encourage hypetrains (so a potential positive ROI), and be a win for everyone involved; adds functionality to the hypetrain product, and shows that Twitch cares about keeping the hype rolling full speed.