Further investigation proves I was mistaken about Charles Olson's notion of the "middle voice". This has less to do with a level of diction or manner, than with a mode of grammar, drawn from Greek originally, in which the verb is between active and passive, & the subject of the sentence is also its object. There are several fascinating google links on this topic. And there may be a tangential relation between Olson's notion & what I've been trying to get at with this notion of "middling" (both ideas are looking for a center of balance).
But the first axiom of The Plumbline School might be -
Poetry = a Happy Medium.