responsetypes.py

    class ReplacementClass(object):

        @property
        def selector(self):
            return custom selector

    def process_response(self, response):
        cls = type(response)
        newclass = type('newclass', (ReplacementClass, getmro(response.__class__), cls))
        class NewClass(ReplacementClass, response.__class__):
            pass
        return response.replace(cls=NewClass)