"I was thinking perhaps as simple as one field "hls URL" and a "go" button. Certainly no json. "

Yeah great idea. You just wrap the json in the background and publish (I guess). You don't need to see the raw format as user.

Here is the thing though. Unless the streamer is self-hosting he wont have a HLS url. If they're streaming on Twitch, YT etc they'll have to use something like restream.io to push their stream to other platforms with their key and the site's rmtp stream url. And if they're already streaming on nostr their stream gets announced anyway by ZS and Shosho (nice bot btw).

Now that I said all of the above I'm not sure if there is much use for such an option. But it can't hurt to have it for people who do have an actual manifest url for their stream.