@bryanlandia I don't quite understand what you are trying to do. Can you send the user to the SAML IdP on registration with a normal login hint and have them register there? As long as FusionAuth gets back the signed assertion, it doesn't care if the user logged in or registered at the IdP.
So you could actually replace the FusionAuth registration link with a link to the SAML IdP with a login_hint.
Or am I missing something?