I think you can have some logic in the theme component to show the login page if the user isn’t logged in.
What I currently do, linking from another site, is enforcing the login flow like this, but I’m not sure if this works if you’re not using SSO:
SITE_URL/session/sso?return_path=/t/slug/id%23reply
Having it in the theme component is better to avoid unnecessary redirects anyway, I’ll try improving it when I get a chance (I’ll comment here if I do).