One thing that may be possible (maybe even a very good idea) if you're interested in learning programming/networking on the old system and on a new system is to do the core of the client on a modern system like Linux/BSD/OSX/Windows and then have that computer communicate (in completely clear text, possibly using something like the IRC protocol) to the 68k Mac.

The things I would look at for examples of this are bitlbee, which does literally this thing (it is an xmpp/aim/msnm client that runs an IRC server that you can connect to in plain text), but it actually acts as an IRC server and Quassel, which has the "core" and "client" aspects.

This gives you more horsepower for the network and encryption bits of your application while letting you create a native Mac ssw6/ssw7 graphical experience on the client end, and also prevents network traffic from needing to go to the 68k Mac directly.