Shoutbox

Yes it could be done, but it involves hacking a few of the games files, but when games are played on a LAN (computers not consoles) they usually keep a stable and high bandwidth to every other computer connected. This is to reduce LAG...however if your game thinks it's on a LAN when it is in fact on the internet it will experience a lot of LAG, unless your internet connection was above 5mbps. So it's not really a good\legal idea...
I did here once I believe someone tweaked how windows read and understood network connections to do this, but even if someone could find out how you'd still have the same amount of LAG...