If it were me, I'd background only the body-region (as Ken says, much easier), and use an SVG image.

The SVG would be tiny, most processors support SVG images (including both FOP and AH), and making an SVG saying "Confidential" in big letters running diagonally, say, would be fairly trivial, with plenty of variations possible (colors, opacity, outlining). Also, it could be managed inline as a literal in your XSLT, if you like, saving you the overhead of creating and managing a separate file.