Totally throwing this out there Mike....
Could the company logo that is added to the PO's also contain a combined image including the stamp?
The only other option that comes to mind, assuming that the "stamp" is a bunch of text, put this into the T&C's that are included at the bottom of every PO. Or provide a link in the T&C's that point to a webpage that references the "stamp".
If this is Buyer and not P2P, then I'm sure we could customize an image to be added to each PO.