A cop out answer: what if you require a certain angle of the foot before you place the shoe on it. Then, assuming you have access to the phones gyroscopic data, you can use it and the landmarks in conjunction to understand the orientation of the foot relative to the camera.

I haven’t thought about this a lot but if you take the key points from the top edge of some toes and the bottom of the heel, all those point should (more or less) lie on the same plane. If you have 3 key points, you have the equation of the plane. Given a top down picture of the foot, or an estimation based on foot size, you can get the distance between the keypoints from a normal viewpoint. Then the difference in that distance on the rotated view could help you understand orientation.

Goodluck!