I finally have time to work some more on WheelMUD. I'm in the process of implementing the Warrior, Rogue, Mage (WRM) combat system. I'm loading it into WheelMUD's GameSystemController, using MEF. The neat thing about this setup, is that third party developers do not have to worry about plumbing details that much. As long as the correct interfaces are implemented, WheelMUD's MEF implementation will load all the different gaming systems/engines, and game attributes from the game's assembly (coded in any language supported by .NET 4.0).
Here's a screenshot of the console with the custom assembly loaded:

The cool thing about this, is that these custom systems will behave exactly like WheelMUD's internal systems.
I don't have any logic implemented yet, but that is my next step. I'm also looking to see how I can get this thing some unit testing love.