Hi,
What exactly do you need?
You would like to get directly portal without logon screen? for that you can use anonymous access to the portal
Using Anonymous Logon to Access the Portal - Configuring the Portal for Initial Use - SAP Library
Or after user logs in , you want to see again the logon screen? for that you can just put in the framework page anything you want without permissions ,for example some page with security zone that doesnt match the user.
http://help.sap.com/saphelp_nw73ehp1/helpdata/en/48/50badbb82971b9e10000000a421937/frameset.htm
Thanks,
Tal