Web Filter Captive Portal

The main goal is to have a captive portal style page appear when accessing sites that we set to "Monitor". The goal isn't to block users from accessing the sites, the goal would be they would enter in their credentials, gain access to the the site for a set amount of time, and we would have full visibility of events while they are on the site. 

I do not see a functionality like this in the current firmware, 5.6.3. I am planning an upgrade this week to 6.0.2. Does anyone have any experience in a setup such as this, or any ideas how we could accomplish something like this?

Thank you.