Just to throw this out there, IndieAuth is a very small addition to OAuth 2.0 which adds identity into the system in a much lighter weight way than OpenID Connect. Mastodon could easily add this extension to return the user ID of the user who just authenticated. The login form on OwnCast would ask the user to enter their server name, and do discovery on the server to send the user there to log in.
I did a talk about how Mastodon/ActivityPub apps can use IndieAuth to accomplish this kind of thing. The video is available -- of course -- on my website: https://aaronparecki.com/2020/09/22/25/activitypub-oauth-2-1