[Zope] using the Session Data Object hooks

The Session Data Object exposes the Transience Interfaces setDelNotificationTarget(self, f ) and setAddNotificationTarget(self, f) which register f to be called when a session object is added or deleted. The registered function is supposed to accept a single argument, item, the object being added or removed. So, how does one write a PythonScript to do this? Or must it be an External Method and/or a product Extension? It should be straightforward enough as it's part of the normally exposed interface.