The classmethod and staticmethod decorators have somewhat different semantics.  Accessing a classmethod with C.cm creates a bound method object which has both __call__ and __get__.  In contrast, accessing a staticmethod with C.sm returns the underlying function.  So there is no opportunity for a customized __get__ method as we have with classmethod.

Also, even if C.sm did return a custom descriptor, the code example still can't be made to work because the __get__ method on property objects is hardwired to pass in an instance.¹  That means that the underlying method must have a "self" argument.  

Another issue is that properties have __set__ and __delete__ methods which could not be made to work without a reference to the class or instance.

To make a read-only class variable, a metaclass would be needed.  While it is possible to write a descriptor to make C.FINE_STRUCTURE_CONSTANT call a function that returns 1/137, there is no way for the descriptors to block the assignment:  C.FINE_STRUCTURE_CONSTANT = 1 / 100

Thanks for the suggestion, but I don't think this can be made to work.  

¹ https://docs.python.org/3/howto/descriptor.html#properties