I would simply add a nameless export alongside every named export if you want it to be resolvable both ways. For example
// named and nameless
[Export("TypeA", typeof(MyPlugin))]
[Export(typeof(MyPlugin))]
// named nameless, again
[Export("TypeB", typeof(MyPlugin))]
[Export(typeof(MyPlugin))]
class MyPlugin { }
[TestMethod]
public void mef()
{
var catalog = new AssemblyCatalog(this.GetType().Assembly);
var container = new CompositionContainer(catalog);
Assert.AreEqual(2, container.GetExportedValues<MyPlugin>().Count());
}