make pdf file and then in adobe acrobat pro:
tools > pages > watermark > add
Thanks, I know that you can apply a watermark in Acrobat, but I'm really looking for a plugin or something within Indesign.
Photoshop has one, it would be just as handy in Indesign.
Create the watermark graphic. Put it on its own layer at the top of the stack on a master page. Turn the layer off and on as you wish.