OK, I got A2SERVER working fine on a Powerbook 180c running System 7.1 via my Fastpath. By default Appletalk/DDP wasn't running out of the box for some reason. I had to type "a2server-setup" and go through the prompts to install stuff. I then typed "netatalk-router-off" and Appletalk/DDP sprung to life! If you already have a router on your network, try that and see what happens.

Disconnecting the Fastpath, I can type "netatalk-router-on" and Appletalk comes up, so I'm guessing out of the box it conflicts with the Fastpath bridging.