As for the backing, I don't think I'd go leather as it tends to stretch and shrink with the weather, when the other side of the screw is in inflexible kydex, it wouldn't work as well as a whole leather or whole kydex piece, it seems to me.
Might be worth a try though as flexiblity across the slide is huge, I found. Can you think of a way to make 2 cups lockable to each other? Perhaps a flap out the side with a snap, that would snap to the cup next door? I think if I could vary the number that were connected I'd love it more.
What about the molle lock from knifekits? Ahhh that's it, the snap on one side of the flap, the molle lock on the other.
If you could make one like that, with a molle lock backed flap that snapped to the back of the one next to it, I'd start figuring out what I had to sell to get ten of them. One could have no flap, just a snap on back and one could have a mollelock behind the mag cup and one on the flap. Then you'd have to lock at least 2 together to leave no flap exposed. but you could add any number you liked to any size or shaped piece of gear from a vest to a belt.
The pivot of one side of the snap to the other should be enough to make it flexible along it's length too. Being held together by the molle web or the belt will keep too much stress from wrecking the snaps i suspect.
Let me know when they'll be ready!