I have this printer too, and indeed, Apple stopped printer support for it with Lion.
After some fiddling with foomatic and ghostscript, I found a simpler method that works perfectly on all my Macs on recent OS (Mac Pro on 10.8, MacBook Air on 10.8, MacBook Pro on 10.8, MacBook on 10.7).
Get hold of the PPD file with the name "LaserWriter 12_640 PS.gz" from OS X 10.6. I copied this PPD from my old Mac Pro G5. The file is in System > Library > Printers > PPDs.
Put that file in your Lion or Mountain Lion System > Library > Printers > PPDs with the other PPD files.
Go to System Preferences > Print & Scan, and add the printer by selecting IP and putting in the IP address.
It will look for the printer on the network, and once it shows in the box, you will be able to select the LaserWriter 12_640 PS in the driver selection.
Maybe you need access to an older system that can run OS 9 (native or in Classic) to use the Printer Setup Utility to configure the printer for IP printing. In my experience it works better if you put in a fixed IP address for the printer using that utility.
Hope this helps. Good luck.