Yeah, it doesn't sound like you should be using an IFrame at all. That isn't going to gain you anything and it's just going to make it harder to access the form elements. I'd go back to an embedded form.
If you can alter the login page, you can hook the submit process for the login form and copy the email address into a cookie using JavaScript. This way, you don't need to make any later database calls -- you are essentially saying, "I stored the email address on Page A, and the only way someone could get to Page B is if their login is correct, so I'll assume the email address in the cookie is the same one they logged in with."
But your dev might not like you messing with the login form, either, even just to grab the email address. You should review the plan with him. The principal choices are (a) you store the email address in a cookie at the time the login form is submitted (without, in theory, affecting the login process at all); (b) he sends back the email on the first page after login (so he's setting the cookie for you); or (c) he builds some kind of web service you call whenever you want to get the user's details.
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