I ended up using oauth2app and it only took me some minor changes in order to get in work with django-nonrel.
The only restriction is that the model classes in oauth2app use ManyToManyFields and this is not supported in django-nonrel, so I ended up changing them to ListFields. This then yields to some minor changes concerning the queries done in the library.
All in all oauth2app can be recommended also in combination with django-nonrel on Google App Engine.