I think I can answer my own query now.
Whether or not the feedback rubric displays depends on whether or not the rubric has been allowed to generate a grade in the gradebook. As far as I can tell, fiddling with that grade (e.g. by overriding it or setting the assignment to be ungraded) has the effect of hiding the rubric containing the feedback.
So my understanding is that rubrics cannot be used for feedback only, but must generate a grade which is visible to students.
Mira