Does Thomas’s metaphysics of the angels preclude or leave open the possibility of Tolkien’s incarnate and sub-creative angels?
According to Thomas, influencing the heavenly bodies is not the only way in which angels can effect change in the natural order. He admits, for example, that angels can and sometimes do assume corporeal bodies, not because it is in their nature to do so (ST 1.51.1), but because in this manner they are able to be of greater service to men with whom they have “intellectual companionship” and whose salvation they help administer (ST 1.51.2). In such cases, however, the angel is not united to the body as its form, as in the case of the human soul and its body; instead, the angelic spirit acts merely as its body’s extrinsic mover (ST 1.51.2 ad 3). A couple of significant remarks made by Thomas on this point are, first, his statement that when angels do appear to men, the bodies they assume are or can be nothing more than “condensed air” (ST 1.51.2 ad3).
Secondly, although angels cannot produce a human body (the angels being themselves incorporeal), Thomas avers that they nevertheless “could act as ministers in the formation of the body of the first man, in the same way as they will do at the last resurrection, by collecting the dust” (ST 1.91.2 ad 1). This would appear to be a particular instance of Thomas’s more general principle that, although angels cannot perform miracles, being themselves part of the natural order (ST 1.110.4), they can nevertheless predispose nature to the supernatural and miraculous working of God, their own knowledge of the powers of nature being so acute that they can perform wondrous albeit natural works. As Thomas writes in one place, “angels are better acquainted than men with the active and passive powers of the lower bodies, and are therefore able to employ them effectively with greater ease and expedition seeing that bodies move locally at their command. Hence again physicians produce more wonderful results in healing, because they are better acquainted with the powers of natural things” (On the Power of God 6.3). Thomas even goes so far as to refer to the existence of an ars angeli or “art of the angels.”
According to Thomas, however, the angelic will cannot command corporeal matter directly, but moves it “in a more excellent way” by moving “corporeal agents themselves” (ST 1.110.2 ad 2). As Collins summarizes, in this way corporeal forms may indeed derive from angelic intelligences, not through an immediate “creative influx” (or direct “emanation,” as Thomas puts it in ST 1.65.4) on the part of the angelic intelligence, but rather through an “eductive process” of “moving the bodies to their forms.” Whatever might have been the case, however, Thomas reminds us that, at least as a matter of historical fact, in the original creation of corporeal creatures no such “transmutation from potency to act” by angelic means actually took place, as Scripture teaches that “the corporeal forms that bodies had when first produced came immediately from God, whose bidding alone matter obeys, as its proper cause” (ST 1.65.4).
 “[C]orpus assumptum unitur angelo, non quidem ut formae, neque solum ut motori; sed sicut motori repraesentato per corpus mobile assumptum.”
 “Et sic angeli assumunt corpora ex aere, condensando ipsum virtute divina, quantum necesse est ad corporis assumnedi formationem.” Dante in the Divine Comedy suggests that the diaphanous bodies of the shades are the effect of the deceased soul giving form to the air surrounding it, making the soul visible as well as giving it organs of sense perception through which even the deceased souls are able to continue communicating with each other (Purgatorio 25.94-105). On the Thomistic origins of Dante’s idea of diaphanous bodies, see Philip Wicksteed, Dante and Aquinas, 223-225.
 “Potuit tamen fieri ut aliquod ministerium in formatione corporis primi hominis angeli exhiberent; sicut exhibebunt in ultima resurrectione, pulveres colligendo.”
 On the angelic knowledge of and consequent power over nature, see Collins, The Thomistic Philosophy of the Angels, 315.
 “Et sic angelus excellentiori modo transmutat materiam corporalem quam agentia corporalia, scilicet movendo ipsa agentia corporalia, tanquam causa superior.”
 Collins, The Thomistic Philosophy of the Angels, 289.
 “In prima autem corporalis creaturae productione non consideratur aliqua transmutatio de potentia in actum. Et ideo formae corporales quas in prima productione corpora habuerunt, sunt immediate a Deo productae, cui soli ad nutum obedit materia, tanquam propriae causae.”