Now I'm confused. I don't understand how you can have preferences that you use to decide what ought to count as a "moral justification" without already having a moral reference frame.

Since we don't have conscious access to our premises, and we haven't finished reflecting on them, we sometimes go around studying our own conclusions in an effort to discover what counts as a moral justification, but that's not like a philosopher of pure emptiness constructing justificationness from scratch and appeal to some mysterious higher criterion. (Bearing in mind that when someone offers me a higher criterion, it usually ends up looking pretty uninteresting.)