I don't think the Activity grader interface should let you do things that are not supported by the underlying activity. What I mean is, that if the Activity grader interface lets you grade essay questions in a quiz using a rubric, then that should work becuase you can grade essay questions using a rubric through the quiz/question UI too.
This is clearly something that needs to be implemented, but it requires someone who understands advanced grading methods and the question engine. The intersection of those two sets may well be empty at the moment.
It might well be possible for Sergio to hack together something that basically works, even if the code quality is not good enough to go into Moodle core. For him, or someone else, to try that, would be a useful learning experience. It would probably give us a better idea of what it would take to implement this properly.