Dynamic Default Roles ?

Fact1: Every user that logs in is assigned a defult role. Fact2: Some users wear different hats when logging onto the database. This requires

            different role assignments bases on the current application and session. Fact3: Each time a user logs on a session is started with an entry in the v$session table.

Question1: Does it put an entry in the v$session table, before the default role is enabled? Question2: Can a trigger be placed on the v$session table?

 If the answers to both questions are yes, we plan to write a procedure to dynamically assign default roles based on the current users application. Some users may be the division head in their own department during the first session/application. The same user may later use a different applicaton with very limited priviledges in another division. The trigger on the v$session will assign the default role dynamically using a procedure which detects which application is in use.

 If the answers to the above questions is no. Does anyone out there have another method, procedure or sugestions on how this may be accomplished.

                                                                Thanks in advance
                                                                Dennis Brewer
                                                                System DBA