Couple things u may already now but I'll mention them anyway:
Is the server binding to the correct ip? I usually use +net_ip ip.ip.ip.ip in the start up line
Do you have the port that gamespy uses open? Port 12300 if I remember correctly. And of course port 12203 for MOHAA itself.
You may also want to join this email list and ask there:
mohaa-subscribe@icculus.org