I was thinking about something along these lines, before I actually built and attached an UQ.
Now, with just a no sew PLUQ, I question the need. I can get the UQ to sit correctly very easily, and it stays in place very well. The problem with a system like this is it would be difficult to adjust, and time consuming to set up. It limits the UQ to one location, so if you shift in your sleep, it can't move with you, or be adjusted to your liking with a quick pull.
You could ease this some by using snap in buckles (like on a pack waist strap), so you can set the UQ once, lock the webbing, and just buckle and unbuckle it. Each UQ you use would need it's own buckles. It would still have the initial set up, and still be difficult to adjust, however.
For a pad, this sounds like a great idea. You still can't move it, but pad are much more finicky, and I'd rather adjust my body than adjust a pad. You just need to figure out how the pad needs to lay to be most comfortable for you.