Following backreferences of unknown kinds in NDB

Interesting question! So basically you want to look at the Contact class and find out if there is some other model class that has a KeyProperty referencing it; in this example PhoneNumber (but there could be many).

I think the solution is to ask your users to explicitly add this link when the PhoneNumber class is created.

You can make this easy for your users by giving them a subclass of KeyProperty that takes care of this; e.g.

class LinkedKeyProperty(ndb.KeyProperty):
    def _fix_up(self, cls, code_name):
        super(LinkedKeyProperty, self)._fix_up(cls, code_name)
        modelclass = ndb.Model._kind_map[self._kind]
        collection_name = '%s_ref_%s_to_%s' % (cls.__name__,
                                               code_name,
                                               modelclass.__name__)
        setattr(modelclass, collection_name, (cls, self))

Exactly how you pick the name for the collection and the value to store there is up to you; just put something there that makes it easy for you to follow the link back. The example would create a new attribute on Contact:

Contact.PhoneNumber_ref_contact_to_Contact == (PhoneNumber, PhoneNumber.contact)

[edited to make the code working and to add an example. :-) ]

Sound like a good use case for ndb.StructuredProperty.