Hi @chris.howell,
I appreciate your patience while we were out for the holidays!
I did more research into this, and apart from the solution I shared above, there is no other way to access/pass through the referrer page via the login. I checked the PHP documentation and do not see an acr_values or other similar parameter to pass in.
I would recommend leaving product feedback here about this: How to Submit Product Feedback or Feature Requests
Thanks,
Mary Beth