I've been thinking about this, because it's something we'd like to do also... and it occurred to me after reading this thread:
Wouldn't a simple solution be to allow the possibility for creating your custom login page in a project of it's own (or in a more complex project), creating a custom username/password field entry and then using an "improved" version of the "system.perspective.login" script function to pass the parameters entered by the user (user/pass) and forward them to Ignition's configured idP? thus starting a session and skipping the normal login screen?
Not sure if I'm missing anything or if this a feasible idea, but we'd like to be able to make a custom login screen and then distribute users to the relevant projects based on that. I've been able to make a version of this, but I'm not entirely happy with it...