It IS possible to use the Classic version of IPNetRouter on a 68k machine. I had made a little DNS and mail server with a LC III once. Just put a Ethernet card in there, get it all setup, then use one of those mac to VGA monitor adapters so that it would continue booting without a monitor attached. and Ta-da! I had even added a sound file in the Startup Items folder so I would know it finished booting.

So if you can get your hands on a second 68k mac that CAN have Ethernet. That would work too.