Multiplayer was scrapped at the last seccond thanks to Akella. If you've seen the credits for the game Toploader is still credited as the MP level designer or something along those lines.
The reason why there has yet to be any form of multiplayer.. Or any kind of proper fixes for SP for that matter, is because the guys over @ Akella are still holding on to the SDK for the game. So no one has really been able to improve, change or add any new features properley.
There are still a some really talanted people who have offered tweaks and minor changes to character models, and such. But like I said, in terms of actual improvments, that this abysmal piece of shit game needs.. At the time of posting we're shit out of luck.