How to Create a Legal Notice upon login in Xubuntu 12.10?
You would first need to create some method to show the notice. The most straightforward way to do this would be to create a simple html page and display it to the user using
If that works you can proceed to add it as a 'Startup application' using the following method: http://xubuntugeek.blogspot.nl/2011/12/add-application-to-xfcexubuntu-session.html
Furthermore you need to be absolutely sure that your legal notice does not conflict with Ubuntu's software licenses (such as, but not limited to GPLv2).