Method is a subclass of FunctionDef, not of Function, which is partially why you can't use it as a function.
> For instance methods, I understand that you'd need to pass an instance
of the object, but for class methods it should be capable of being
"free-standing"...no?
This is not quite correct. As a result of the design of SuperCollider's class system, 'class methods' can be viewed as instance methods on a meta-class. So you need to pass an 'instance' (the only instance) of the meta-class.
-Brian