i currently do the first thing to check the live activated extensions endpoint. however the issue of vodcast came up, and only way i know how to do it is as you suggested. however im afraid of the rate limits. say a streamer has 10,000 people hit a page with this extension, as far as i know live activated extensions isnt rate limited so it was safe, but new twitch api definitely is rate limited (30 for non bearer 120 for bearer)?
I also suggested a long time ago for the onContext - context object tell us wether the streamer is live, hosting, vodcasting but it was never added.