Leave a message while locking the screen.

In Ubuntu 10.04, after locking the screen, someone else visiting my desk can leave a message. When I arrive and unlock the screen, I can read these messages. Could this also be implemented the other way around? So, when I leave, I press the 'lock-screen-with-message' button, type a comment like 'I'm at the dentist till 14.00h', or 'Present at group meeting in room x.y.z', so that the guy visiting by desk, can see where he can find me, or know when I return.