I just wanted to throw out there that I'm currently doing loaded skater squats (aka air lunge) as my main squat movement. Skater squats have much lower mobility requirements than pistols. If you want a more upright torso and stability, Bulgarian split squat and regular lunges. (One downside of these variations is that you don't go below parallel, but I do them while standing on a ream of paper or two, which solves this problem. This works with BSS, too, and if you're confident the block won't slip, it also works for rear lunges.)
side note: I read in TNW that Pavel doesn't recommend loading the "airborne lunge," but doesn't say why. (I am particularly confounded b/c my kindle version of the book seems to have the airborne lunge section left out, so I don't even know if the exercise I know as the air lunge is the same one described in the book. Also mentioned but not covered are renegade pistols, and fire in the hole pistols.) I've not been doing them for many years and I'm not a coach; I can only say that the exercise feels great to me. The only risk I can imagine is low-back stress due to lumbar flexion at the bottom, but afaict there's more low-back flexion with pistols, so I'm at a loss.
