Thanks for your reply. I’m not certain I’ve explained my suggestion well, because I would have not expected it to be such a major change.
I’m looking for a URL of the form
https://discourse/login_sso_dispatcher?provider=google
which would have the same effect as clicking on the relevant login button (but the action that takes is shrouded in javascript).
Implementing an SSO provider on the main site might be possible but would introduce a hard dependency on that site which might be worth avoiding.