If a user already has a session, and navigates to an OpenID or SAML protected website, he will not be prompted to login (i.e. that's what SSO means).
If a user does not have a session, and navigates to an OpenID or SAML protected website, he will be redirected to the Gluu Server. You can send custom OpenID Parameters when you redirect to the authoriztion endpiont. In the Person Authentication Interception script, you could pick up these extra parameters and do something (like route to a specific login page).
So it's definitely possible to do it... but you have to know a little bit about how to write these interception scripts.