Yeah, the command line arguments are very basic. (and the code to handle some of them is incomplete, actually)
The most i have found regarding multiplayer sessions are these.
/net: -- (Including the : ) it will start the game and open the multiplayer dialog directly up - where you pick a connection type.
/net:sigs -- This is the equivalent of opening the game, and going Multiplayer > SIGS. (It will do this at game startup).
You can't do /net:tcpip or anything like that, it checks string literals exactly like "/net:", "/net:sigs" -- it doesn't parse any data after the colon, and those are the only two strings it checks regarding the "/net" argument.
There are some other arguments that were either removed, or don't do anything in the first place (uncomplete).
/map:name -- supposedly loads map name.map. It doesn't quite work though. If you omit the :name part, it shows newworld.map.
/noshell -- starts the game directly to the game (with the gray borders) instead of going to the menus. Still, pretty useless though.
/noview, /playback, /cmdlog -- These were never finished, the code simply checks for the presence of these switches and throws the result away (subroutine ends).