Coincidentally, I just ran across this in the Summa. Its not really addressing the All Powerful Hand of God devotion but the propriety of worshiping Christ's humanity. However, it is somewhat analogous.
"I answer that, we may consider two things in a person to whom honor is given: the person himself, and the cause of his being honored. Now properly speaking honor is given to a subsitent thing in its entirely: for we do not speak of honoring a man's hand but the man himself. Adn if at any time it happen that we speak of honoring a man's hand or foot, it is not by reason of these memebers being honored of themselves: but by reason of the whole being honored in thme. In this way a man may be honored evin in something external; for instance his vesture, his image, or his messanger.
The causse of honor is that by reason of which the person honored has a certain excellence: for honor is reverence given to something on account of its excellence, astated in the Second Part [of the summa]. If therefore in one man there are several causes of honor, for instance, rank, knowledge, and virture, the honor given to him will be one in one respect of the person honored, but several in respect of the causes of honor; for it is the man that is honored, both on account of knowledge and by reason oof his virtue.
Since, therefore, in Christ there is but one Person of Divine and Human natures, and one hypostatis, and one suppositum, He is given one adoration and one honor on the part of the Person adored." - St. Thomas Aquinas, Summa Theologiae
I would say that in an analogous way we can apply this to the All Powerful Hand of God devotion. The hand of God, which is not really a physical hand, is a sign of power and his power is one of the many reasons we adore him. However, the adoration is for the Person who is God. In Thomas' arguement we see that the honor given an aspect of a person is given for the ultimate purpose of honoring the actual person.