Rumor has it that the interface was considered briefly for one of the circles of Hell in Dante's Inferno, but concern over censorship caused it to be removed at the last minute. However, seeing as how you've demonstrated the proper humility before its great hardcoded evil..
You might be able to get away with what you're trying to do by editing the mask for the room you want to modify (I assume it's the main hall) and removing the mask color for the multiplayer room. However...I don't think that the main menu actually has a separate mask template (The mask files are denoted by "-m") so you might be better off removing the multiplayer pilot link from the player select screens, both at the start of the game, and in the barracks.
That being said, I remember that there is an alternate method in the code for specifying the location of the mask color, so it may or may not disable the button completely to remove the button color from the mask for a 'room'.
