I've been toying with the idea of using half of one of my spare plate carrier vests ($20/each at a gun show) on the back of the front seats to mount pouches. I just strapped the carrier on the driver's seat and tried it for a while. What I found was that I probably need to put a piece of 1/4" plywood inside the carrier to stiffen it, since it tends to sag away from the seat back even with nothing on it.
I've since taken it off, so no pictures, but as soon as I can order some more fastex buckles, I'm going to put them back on with the plywood and see how it works. I also like the idea because the rear window tint makes it hard to see the stuff on the back of the seat.
I'll post back here when I get them back on.