Another one of the arduous restrictions ruby places on you is that it doesn't allow you to dup singleton classes.
Evilr is your friend here, offering a dup_singleton_class method that gives you a duplicate of the singleton class.
The stupid thing about this is you can get a dup of a singleton class in ruby easily just by cloning the object and getting the singleton class of that clone, so the restriction on duping singleton classes makes little sense.