I pressed the voice search button on the Chromecast with Google TV remote and said "White House Correspondents Dinner" about half an hour before it was to start. The response was "WatchList" "Watched it?" and the up and down thumbs. No indication of where or how to watch.
Using my remote, I then went to the YouTube app and searched for the C-Span channel, and there on the channel it was, marked Upcoming and just a few minutes from starting.
Hey there JJPrats,
I apologize about the late response, thank you for being patient on this! That is an interesting observation, it makes me wonder if the Google Assistant isn't aware of YouTube Live Streams from the Home Screen. If you have the time, would you be able to submit feedback via Google TV to let Google know? While they won't respond back to you directly, this is the best way to let them know about bugs.
I look forward to your response, and if you have any other questions or concerns for me, please let me know.