This sounds possible if you can do some customisation of forms and add some logic to the below theory:
User connects to a captive portal page on the Clearpass which automatically logs on with a username and password. This would be an auto-generated account which is specific to this user. (you would probably need some logic here to auto-generate an account with a timestamp or somthing to make it individual). Also the MAC address of the client is recorded and added to the device database. The 'free 5 mins' account is set to be deleted and disconnect the user after 5 minutes. The MAC account has a 1 hour lifetime. The DHCP lease of the network would also need to be short.
Once the 5 mins expire and the free account is deleted, the user is disconnected and would need to reconnect. MAC auth should then pick up the user which could assign a different captive portal profile which points the user to the login/register page to create an account which is valid for longer.
This sounds ok in my head. You would have to work out whether all this is technically possible.