Being it Monday morning, I still may be dense.
So you are trying to to eliminate logging in and just using some readily available environmental information to identify the user? Correct, so far?
If so, why is there a login page to being with? If you aren't going to ask the user for his or her password, why is there even a need to show a page that says "Here's what you are going to log in as, you don't have a choice."?
Guess I'm still not understanding the scenario as it doesn't seem to make much sense. I just may need more
coffee, but it sounds like you are trying to take an authenticated system, turn it into an un-authenticated systems, but still leaving a lot of the authentication mechanism in place, and tripping over it.