I believe "public good" is used in Dignitatis Humanae rather than the "common good." Can it be identified with the classical notion of the common good, or is it a subordinate good, akin to peace? One could argue that peace is an instantiation of the common good, in so far as it is not merely the absence of strife and violence (and injustice), but caused by the members of a community abstaining from such acts. On the part of the majority, such acts may not result from virtue, but from the fear of punishment--nonetheless, it would be the barest instantiation of the common good, as it is understood in Aristotle and Aquinas, since it can only come about through the compliance of the members with the laws, and the enforcement of the law by the public authorities.
I'm going to have to read Finnis again on the instrumental common good and see how he explains it.