Goddamnit. Had to look it up.
Dante mentions “là dove la cosciasi volge, a punto in sul grosso de l’anche.”
In Mr. Kobayashi’s quote, the line is translated as “the point at which the thigh revolves, just at the swelling of the hip.” In the page he linked to, it is translated as “the point exactly where the thigh begins, right at the haunch’s curve.” In Longfellow’s translation, it’s “where the thigh revolves, exactly on the thickness of the haunch.”
The use of the word “haunch” (“a buttock and thigh considered together, in a human or animal”) certainly seems to point in the direction of the Devil’s ass, rather than, say, his stomach. So it seems exceedingly likely that Dante is, at this point, standing on (not in) the Devil’s ass, rather than, say, hanging out in his navel.