States in drawing headers can be used to solve this. This functions makes it possible to - page by page - to choose a certain state, ie Confidential, Test, Preliminary etc for the relevant pages.
In order to have the states to choose from, the drawing header must be designed with states. Open the drawing header in symbol design. Choose the State icon to enable states. Go to layer 2 and place the text CONFIDENTIAL (maybe with large letters, rotated 45°). State title can be written in the dialog symbol settings. More states can be made by placing other texts in other layers.
There are more information on this subject in the manual.