I have a LaserWriter 12/640. In Classic printing, there was an option to print to the Face-Up output tray, which provides a straight through paper path. (Important for printing labels.) When I switched to Mac OS X, I found that option was missing. The solution was to make a copy of the standard PPD file and add a fake "paper tray" that acts like the Multipurpose tray but prints to the Face-Up tray. Use the custom PPD when creating the printer and just select "Multipurpose-FaceUp" when printing.
The PPDs for Apple printers are in /System -> Library -> Printers -> PPDs -> Contents -> Resources -> en.lproj. First, I duplicated all the lines that referenced "Multipurpose" to create a "Multipurpose-FaceUp" input tray. Then I added the following line to the Postscript code that selects this tray:
1 dict dup /OutputFaceUp true put setpagedevice
(I found this line elsewhere in the PPD.)