A downside of shoulder height sockets will be the temptation to, if a cable is too short, stretch is between (eg) light on a stand, and the socket, leaving it dangling and unsafe.
Could you do the ceiling pull down thing with a normal sprung loaded dog lead bolted and a standard 4 gang block?
I'd maybe put some sockets on the ceiling too, especially if you're going for a hilite system, and also bear in mind with your socket positions that you might want to use bits of wall other than your main backdrop, (bare brick etc) that you wouldn't want to have a plug socket in the middle of.
A socket behind the backdrop would probably be useful too, for rear lighting / props.
And don't forget to budget in some cash for 13A extension leads, in varying lengths - eg 2, 5 and 10m lengths going to dual gang sockets, and also a ton of 4 gang blocks - having plenty of cables easily to hand (get some kinda cable hanger for them) helps you to maintain good and safe practice with mains cabling. Maybe buy a 50m reel of decent black 13A cable and make your own, would probably be cheapest...
oh, and it goes without saying to make sure that the whole thing is on it's own ring main with a fast tripping RCD, valued at close(ish) to your likely maximum load.