Do less goal with slope of 0 where you input 1 if you watch youtube before noon, 0 otherwise?
Something using rescuetime integration (not sure how that’d work, just spitballing)?
It should also be trivial to write a Tampermonkey script to keep track of the amount of time you spend each day on a list of domains before noon, and post a datapoint if that number exceeds a small limit (eg 5 minutes to make sure you don’t just accidentally click something and get immediately derailed).
I think to get it to be easily configurable and usable by as many (non-nerdy) beeminder users, you’d need to make it as an extension. I have no experience in that area, so I don’t know if that’s substantially harder.