"Of note here is that we are not proposing a function to retrieve members of namespaces..."
I believe "Automatic Class Register" for load/storing frameworks, may be a good reason for members of namespace ( obviously filtered ):
I need to get all classes derived from "Widget" (called T in this sample) in the namespace "MyFramework" and all of it´s inner namespaces, that has funcion "toStream(ostream&)" declared.