Do we have any meaningful examples to show that this is desired and useful? 

The primary use case for classmethods is to serve as alternate constructors that return new instances.  That doesn't really lend itself to extending in a subclass.  User's can already call a parent class directly without super(), but I don't think I've ever encountered a single example of someone doing so with a classmethod such as dict.fromkeys() or datetime.fromtimestamp().