Your example was valid, and I think your workaround should be fine.  Do you have a use case for this, or is this question motivated purely by curiosity?

It's fine to modify the __annotations__ dict after the creation of the class or module.  It's code that modifies "__annotations__" from within the class or module that is disallowed here.  Similarly for dataclasses; once it creates a class object, it can explicitly set and / or modify the annotations dict on that class.