Hi Oliver,
In MEF's discovery model, only types that have exports are considered "parts". This is because although MEF could find imports on types like the one above, there's no way for a composition container to create instances of them, so instead it ignores them until a method like SatisfyImports() called.
MEFX is only a thin layer over MEF's regular discovery/catalog system, so it maintains the same behaviour. What you're seeing here is expected.
What situation are you aiming to detect, using MEFX in this scenario?
Hope this helps,
Nick