In 2006 Microsoft released a modified version of the source code along with most game assets (i.e. everything needed to compile the game without networking support and without all the art) under a Shared Source licence in order to demonstrate its XNA Build system. Source is available from Microsoft's downloads site.
Wolfman's Missions has pages and pages of Missions. I had to use IE and double click on links to navigate the site.
mechcommander 2 downloads from wolfman's missions
Mech Labs had a vast number of downloads available. Over 70 new Single Player missions and 18 Campaigns, plus other files useful to MechCommander 2 modders...
mechcommander 2 downloads from mech labs
Shared Source Limited Permissive License for use of MechCommander® 2 This license governs use of the accompanying software. If you use the software...
mechcommander 2 eula
MSDN Webcast: MechCommander 2 Mods (Part 3 of 3) (Level 100) - Introduction and Game Design Decisions.
mechcommander 2 mods - msdn webcast (part 3 of 3)
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