xsl fo watermarking question

Hi all. I'm trying to get a watermark to appear on an XSL-FO document. I've
tried using a background image, and that works fine for plain text, but the
documents I need to render use a lot of TIFF images with white (i.e.
non-transparent) backgrounds.

Is there any way to layer a watermark "over" all the elements of the page
instead of under them? The watermark would have be be partially transparent  so
that you could see the text and graphics underneath.

FYI: I'm using XEP 4.1, and am open to using XEP extensions if necessary.

Rick Geimer