The DNN signin module has a custom redirector which can be set to redirect all users on login to a specific page.

Open the User Accounts Module  from the ADMIN PAGES then select the User Settings  MENU ACTION.

You are look for:  Redirect After Login

This can be used to force all your users to custom page ... from there on you will need a small custom module I guess that queries a database table and displays an appropriate message - otherwise it forwards the user on to a starting page.

Be careful however - since this does not work if the user has select to REMEMBER their login in a cookie.

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The other option would possibly be a SKIN object that sits on your skin as WATCHES for a user login.
When it sees a login it could test the database field - and redirect to a custom page - AND also I guess handle the capture of the redirect back to the current user selected page when the appropriate check box has been checked.

Westa