I'm not entirely sure that this is an issue with MefContrib or not, but the exception I get appears to be coming from the Composition side of things.
I was wondering if there was perhaps something within MefContrib that would cause it to have problems running with Mono?
I have posted on StackOverflow about this, but I couldn't find a MefContrib tag:
Mono doesn't like DirectoryCatalogs where dlls in the directory do not include any exports/imports. This means that the bin directory causes problems. It's simple enough to work around and it's a bug for the Mono guys rather than here.