Thanks, Rick. That's great - I also have a CLP-300, and have been trying to resolve this problem since upgrading to Maverick Ubuntu.
CUPS-PDF seems to be a nice adjunct to Gnome Manual Duplex (although I have only done very little testing at this stage) ..
Install CUPS-PDF (a package if you use Ubuntu)
Create a new printer on your PC:
System -> Administration -> Printing -> Add
Choose 'Generic CUPS-PDF Printer' for the device
Accept default description of 'Generic-CUPS-PDF-Printer'
Print all your duplex documents using this virtual printer - it will create a PDF file in a 'PDF' folder in your home directory.
Now, from a computer preferably near the printer, use Gnome Manual Duplex to send the .pdf file to your printer.