still relevant:

> If the type lookup fails (perhaps because something was included in +propertyKeys which is not an @property), what happens?
>
> We should have a test verifying whatever our expected behavior is.

You mean if subclasses overrode `+propertyKeys`? I'd say no implicit transformers and users would have to go the old, key-based route.
Alternatively, there could be something like `+[MTLModel objCTypeForProperyWithKey:]` & `+[MTLModel classForProperyWithKey:]` but I can't come up with a case were a property couldn't be used from the top of my head).