I was looking at the CompositionHost API and I have some suggestions:
Because you don’t expose the container I think you should expose the support for
GetExportedValues (many scenario use it instead of using the Import attribute).
I suggest adding the following APIs:
static IEnumerable<T> GetValues<T> ()
var plugins = _container.GetExportedValues<T> ();
static T GetValueOrDefault<T> ()
var plugin = _container.GetExportedValueOrDefault<T> ();