This isn’t really possible still, and isn’t likely to be made possible in any first-party way. What you could do is stand up an external identity provider that you stand on your own, which could have its own routing/redirect rules in place for conditional logins.
An update regarding our little project. We have managed to successfully redirect users from a perspective project named “Log in” to the customers own Perspective project using Auth0 as idP and setting up rules in the auth flow including meta data.
The flow is as follows:
Users goes to our.domain.com/login
He sees the Log In project, selects “Continue to log in”.
The auth0 login form is shown asking for credentials if not already authenticated, otherwise straight to the project defined in the users meta data.
We would like to skip the “Continue to login” step, but as I understand the auth process needs to be started from Ignition, we cannot send our users straight to the Auth0 login form.
Can the login screen be customized to our branding?
Is it possible to start authentication from a WebDev project? Auth0 has Python libraries to download.