Is there any way to make a graphic in SAPScript behave like a watermark?

I want to put a watermark in our Dunning letters that go out to vendors that says that this is a reminder notice; don't refill the purchase order. I want it to lie behind the text of the MAIN window in the purchase order layout set. When I try this via the BITMAP command, however, the graphic covers up any textual data behind it. Since I'm using a BMP (TIFF's would do this too,) transparency is not, I believe, an option. Is there any way to force a page window into the background?

Thanks,

Eric