Created attachment 357809 [details] ppd file
Created attachment 357810 [details] printed test page
Created attachment 357812 [details] printed test page
Filtering the CUPS test page through bannertops and pstops with that PPD gives me correct PostScript output, which renders correctly on an HP LaserJet 6MP when printed to that. The problem is with the PPD file: it contains incorrect information about the imageable area for that device.