Once again, thinking out loud.
Consider the Platonic theory of participation of matter in form.
It's erotic causality, not efficient. Material things desire eternity, and any material thing gains its formal qualities per its desire for a form, while the form remains unmoved. However, this remains a one-way causality.
The reality of quality works similar to this way. To halt the infinite regress of nominalism, I argue that universals are real but non-existent, and they become instantiated through particular interactions. The "universals" in this sense can be understood as principles of unity that are on the border of the intelligible (=generic or specific unity), and any determinate existence invokes a specific unity that we may categorize as one of these universals. This approach solves the logical problem, although it is counter-intuitive.
The ultimate structure of unity is the cosmic and existential limitation on possibility. That is, what is the structure of possibility qua possibility. This is not to ask what is possible, mere possibility to be this or that. Rather, it is to ask about the structural relations of possibilities. That is, what is the possibility of this relating to that, and what are the possible relations? This is an abstract discussion whose first level of concretion is when we say "potentiality" rather than "possibility." Possibilities are logico-mathematical realities, whereas potentialities are existent possibilities. Not all real things are existent, and existence constrains possibilities to the condition of particular existence or existence more generally.
Let us tweak the strict Plato model a little more. What if the forms slowly evolve over cosmic time-scales? Then we get much closer to the standard process-metaphysical view of the "laws of nature" that govern the interaction of things such that they come into unity. Coming into unity is to have determinate possibilities rather than the raw flux of chance. Mathematics is a particular determination of pure possibility that becomes determined once we choose its axioms. Existence is another order of determination, although online mathematics, it cannot be rid of pure chance. It scarifies the pure crystal clarity of mere reality for the existential possibilities that in concreto become potentialities. ("Existential possibility" is the ontological term, whereas "potentiality" is the ontic; the latter presumes determinate laws of nature.)
Were we not to talk of the reality of quality? Real quality is just a unity that may be achieved through particular interactions. Insomuch as a unity is a determinate structure of possibility, it has a reality separate from any particular existence. Existence longs for unity as matter strives for form, though the potentialities for any particular unity is limited by nature.
What do we have here? A realist re-description of nature that explains the reality of phenomenal qualities. Ok, "hints" rather than "explains," but I have just unburied myself from my workload for a moment....