models.py

# From basket app
from core.utils import registry
from basket.abstract_models import AbstractBasket

if "Basket" not in registry or registry.get("Basket")._meta.app_label == "basket":
    class Basket(AbstractBasket):
        pass

overriding_models.py

# From the overriding app
from basket.abstract_models import AbstractBasket
from core.utils import registry

class OverriddenBasket(AbstractBasket):
    pass
registry.register(OverriddenBasket, "Basket")

utils.py

class Registry(object):
    blah blah registry code

registry = registry()