The constructor of ExportFactory takes a function that returns a tuple containing the export and another function that release the export. So you can do something like that:
static Tuple<ISomething, Action> CreateMock()
{
return new Tuple<ISomething, Action>(new MockSomething(),
() => Console.WriteLine("Releasing..."));
}
and inject it to your other class' constructor with something like:
var obj = new OtherClass(new ExportFactory<ISomething>(CreateMock));