Here's one which is actually Duo-based. It requires having a unix-like OS at the other end, though. That would be possible on a 3400 (thus OS X via XpostFacto, LinuxPPC, unofficial Ubuntu-PPC, YellowDog, NetBSD, etc), or it may give some clues towards making it happen with OS 9.
Adventures in Networking a Powerbook Duo.
other links:
PB180 online via LocalTalk?
Mac 128k, BBS & Internet?
Netatalk and an SE/30?
An idea: TCP/IP to serial bridge (modem emulator)
Mac ED FTP server via SLIP
Mac M0001 on the internet!