So it looks like the IPX tunneling doesn't work cross platform. I can get to Mac copies of MoO2 to connect, or two Windows copies, but not across platforms.

It looks like the reason is the ports. The Mac version appears to listen on port 2131, whereas Windows listens on 213 and 55247.

Is this a known issue? Is there a solution? If I want to do this, I suppose I can try to tunnel port 2131 to port 213 on my Mac. Unix systems require root access to bind to ports below 1024, which is likely why the Mac uses 2131, but you'd think the Windows version would be updated to cooperate.

You can specify the port in the Windows' .conf (dosboxMOO2_client.conf & dosboxMOO2_server.conf) files. GOG's multiplayer launcher inserts the client IP. Depending on how it does that it may overwrite the port. In which case make a copy of the .conf file and create a new shortcut using that file instead.