Hi Ben
I think you are on the right track of using the Assignment and the point system. I would then add Outcomes (with specified scale) if they still want to have the "satisfactory/exceeds expectations/etc." type of thing. That way they could put both the points and the "feedback" in at the same time. This will also show in the users gradebook.
You could take the point value of 1, 2, 3 ( is not not 0, 1, 2?) and use the "multiplicator" (if I understand how it works) of some bizarre decimal value and convert the points to the final point value you want. It may not come out to 88, 95, and 100 precisely so you might have to change your values to fit Moodle.
I make my own scales so much of the automated grading does not work for me anyway.