OS9 is so modular that there would be no problem in
writing the device
manager and device drivers for a netwoek drvice. I think ethernet would
have had far to high a data rate, but there are plenty of slower
alternatices.
Sorry for delayed response, vacation intervened.
http://www.cloud9tech.com/ (no affiliation, other than as a highly
satisfied customer) does market DriveWire for the CoCo3 running
NitrOS-9. I have not (yet) used it, but they claim 115,200 bps on a
CoCo3. There is a corresponding server that runs on the (Windows/Mac
OS X/Linux) other end of the serial wire, and they claim TCP/IP and
several dependent services (telnet, MIDI streaming to the server). So
Tony's suggestion more or less already exists, with the assumption
that you don't mind a modern-ish PC acting as an external ethernet
<-> serial adaptor.