I haven’t got a site with SSO to test, but I think you should be able to use an API key generated in the admin panel to authenticate to an rss endpoint?
Something like https://meta.discourse.org/t/notify-moderators-when-a-user-reaches-tl3/67071.rss?api_key=blahblahblah
works for me on a non-sso login-required site without logging in