It is possible to use MEF through WCF, I have worked on a similar project in the past. You could expose functions from the plugin DLL's through MEF assume the implementer of the MEF interface is implimenting a function that starts a new WCF service with it's own end points, then call that function from the main service.
See some sample projects below, although none of them are doing what you are asking it is possible that the MEF plugins could have a service host that's starting their very own service with their own end points.