As someone who played a _TON_ of multiplayer MOO2 back in the day (I still consider it one of the greatest games of all time), I'll offer the solution I used (note: this is not DOSBox-specific, and actually I haven't tried it in DOSBox, in case anyone is wondering).

In addition to dial-up and the long-since-defunct WON, MOO2 supports networked play using IPX/SPX. Kali is a service that essentially tunnels IPX/SPX over the internet, so that you can play old LAN games over the Internet. Note that there are two executables for MOO2, a DOS one and a Win95 one--these are incompatible with each other, so everyone in a game must be using the same one (the DOS version works fine under certain versions of Windows, but not all. YMMV). Also note that the Win95 executable has drastically worse performance than the DOS one for multiplayer games.

Now, here's the catch: Kali is not free; last time I checked, you paid a one-time amount of $20, and you get a lifetime membership that you can play any game you want through it. I believe there's a demo version as well, if you want to make sure you can get it to work before paying any money. There's also a small but active community dedicated to playing multiplayer MOO2 games over Kali, who I'm sure would be happy to help you out.

Anyway, here's a couple links to check out if you're interested:
4381's newbie guide
The Kali MOO2 League[/url]

DosBox itself also has tunneling IPX support built-in. See the multiplayer doc in the help section.

The official 0.6.1 version still doesn't emulate IPX although the CVS supposedly, which who knows where does it is hosted, supposedly does.