After re-readiing the title: "Is the 45 degree bodyweight hack squat foundational to foot rehabilitation?" I have another comment.
To answer the above question, as stated: no.
Edit: see below; but if the hack squats help you, they are worth doing.
If you really want to know about the foot, go search "Gary Ward Aim." He is the master of all things foot-related imo.
Read our blogs and dig deeper into the unique assessment and treatment methods of AIM and how pain and body movements are correlated.
findingcentre.co.uk
The only "main structure" of the body that has more joints than the foot is the spine. The foot is supposed to move (and so is the spine, for that matter; but that's a whole thread on its own). It's not supposed to stay in an arch. The purpose of the arch (if we want to talk about it this way) is to give the foot range of motion to spread into to absorb force. It does so by
pronating and flattening, then re-supinating when you lift it. Pronation, hip and tibial internal rotation are also linked to dorsiflexion of the foot. Have trouble with ankle mobility? Look at the other things. Pronation is coupled with internal rotation of the tibia and hip, as well as pushing into the ground. If you watch sprinters, for example, you can clearly see this.
Gary Ward's book,
What the Foot? is not cheap, and it requires some anatomical knowledge, but it changed my perspective on movement drastically.
As an interesting side note/tangent: internal rotation is how we push in general; this includes the upper body. All your major "pushing muscles" are internal rotators. If you perhaps experience what you consider to be "impingement" symptoms with internal rotation, you might just not have enough of it! You cannot move into a place you already are.