I've always read/been taught/believed that the hook should be thrown with the thumb and palm inward- as if grabbing a vertical pole, as you describe., Also, my understanding of Corbett's "invention" of the left hook was that it was to protect the outside knuckles, as it was customary at that time (and I think depictions of bareknuckle fights will bear me out on this) to land a jab with the thumb up, hence putting the small knuckles at greater risk.