Good real-world uses of metaclasses (e.g. in Python)
SQLalchemy also uses them for declarative database models.  
Sorry my answer isn't very different from your example, but if you're looking for example code, I found declarative to be pretty readable.
I'm learning about metaclasses in Python. I think it is a very powerful technique, and I'm looking for good uses for them. I'd like some feedback of good useful real-world examples of using metaclasses. I'm not looking for example code on how to write a metaclass (there are plenty examples of useless metaclasses out there), but real examples where you have applied the technique and it was really the appropriate solution. The rule is: no theoretical possibilities, but metaclasses at work in a real application.
I'll start with the one example I know: 

Django models, for declarative programming, where the base class Model uses a metaclass to fill the model objects of useful ORM functionality from the attribute definitions.

Looking forward to your contributions.