Yeah these are very smart things you say.
I actually did the glute iso hold today, 2 times per each side for about 20 seconds, but I haven't really paying attention to what muscle is dominant. I'd say it's a hamstring-lower part of the glute kind of thing for me. In fact, that's where I usually get sore when I do hip dominant activity.
You remembered me two other very illuminating things:
1) When I deadlifted the 1st EVER deadlift when I was 18 years old (and I can vividly remember that like it was... today

) I used to lift the bar from the floor with 100% (or whatever) quad involvement. I used to just bend at the knees exclusively (knees going forward) with the hips TOTALLY de-activated (torse perpendicular to floor, upright) and I was feeling totally "natural" that way.
People were saying to me "that's not how you deadlift, you need to keep your back straight and use your hips" and I just couldn't even conceive what "using the hips" means at all. I wasn't even aware you can do that movement.
and
2) When I jumped at a basketball hoop for the first time ever (15 years old - and yes, I can remember this clearly as well) the jump was off one leg with the knee going out a LOT and all quad - jump was ~16 inches). Just over time, jumping and jumping and jumping at the rim the technique kind of started to change on it's own without any strength training or anything... I think the body just recognized a better movement pattern automatically and just sticked with that. Interesting.
I should film a right leg jump to see how ugly it is and how much quad dominant it is.
But anyway, I was thinking of dumbbell BSS or step-ups as SPP exercises with deadlifts as assistance or hip thrusts. Single leg box squat sounds good though - the more in front the foot, the more PC involvement so I can control that. Do you talk about pistol box squats variants or barbell box squat variants? (I assume barbell).
PS. This quad dominance I think it's keeping me from doing things like snatches and power cleans as well. The bar is in front of the body and I automatically feel the "need" to lean forward, load the knees (quads) and jump which wrecks everything.