Marathon requires only System 6.0.7, and I think you could get NetLink/TCPSerial to run on there by adding the right stuff here and there...
The real limitation of that being that it's only one-on-one; it'd be much better to have a real client-server AppleTalk-in-IP VPN solution so we could all link our old Macs into a huge AppleTalk network... yet another thing I'm not quite a good enough programmer for.
With that, we could probably live your dream of multiplayer Chuck Yegar craziness.
The real limitation of that being that it's only one-on-one; it'd be much better to have a real client-server AppleTalk-in-IP VPN solution so we could all link our old Macs into a huge AppleTalk network... yet another thing I'm not quite a good enough programmer for.
With that, we could probably live your dream of multiplayer Chuck Yegar craziness.