Poking around a bit more, it appears that there is code, even if this stuff isn't installed:

http://www.opensource.apple.com/release/mac-os-x-1062/

Recent Darwin builds do have AppleTalk source, and it has been updated since 10.5.8. Still I would be surprised if I could just compile it and run. Probably quite a bit easier is that the tarball "network_cmds" does include slattach, the SLIP command.

It will need to wait for next weekend, time for bed!