I've done something like that on my project (http://mefistonet.codeplex.com/).
In the sourcecode, you'll find a proof of concept (dev/V0/Mefisto4), where I use MEF and MVC4 together, allowing me to have some plugins, working without restarting the website.
The magic comes from my rewriting of the controller and view engines.
Just take a look when you'll have a moment ;)
arsenikstiger