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I like your suggestion of not providing __call__(), as I don't see a way to make it work with arbitrary underlying descriptors, and neither classmethod nor staticmethod is callable.
In terms of usage, I think this approach will be OK, as in practice I expect @classmethod, etc, will be applied to the initial method definition as appropriate, so they won't need to be inline except in test cases like these.
Additional test cases needed:
 A().add10class(5)
 A().add10static(5)
 A.add10(A(), 5)
All three should produce 15 as the result, and print the same thing as the following:
 A.add10class(5)
 A.add10static(5)
 A().add10(5)
A test method that uses a partial instance as the callable would also be helpful in assuring the edge cases are covered.
I believe it may be necessary to have a __get__ method something like:
 def _make_unbound_method(self, cls):
 def _unbound_method(*args, **kwds):
 call_keywords = self.keywords.copy()
 call_keywords.update(keywords)
 return self.func(*(self.args + args), **call_keywords)
 _unbound_method.__objclass__ = cls
 return _unbound_method
 def __get__(self, obj, cls):
 get = getattr(self.func, "__get__")
 if get is None:
 if obj is None:
 return self._make_unbound_method(cls)
 callable = self.func
 else:
 callable = get(obj, cls)
 if callable is self.func:
 return self._make_unbound_method(cls)
 return partial(callable, *self.args, **self.keywords)