How can other developers help and participate in building, maintaining and operating the MUD? I've always wanted to be able to contribute to it. Am I correct that the source code isn't public?
You are correct that the source code isn't public. There are two levels of source code, the in-game code (quests, regions, etc., written in LPC in our case) and the MUD driver code (C). The in-game code has been leaked a few times by some of the developers, and so many of the people that want it have it by this point.
The MUD driver we started with (TMI-2 1.1.1 on MudOS v22pre8 if you want to look it up) is heavily modified now, but the original source is out there.