Actually,l asomething like they have on the new tahoes would work just as fine, my mom got one recently, and when the rear seat folds down, the headrest automatically folds back so that it is positioned on the seat back.
I had this in my old Blazer, and it worked well. When the seats are folded, the headrest flips back so that while the seat back is horizontal, the head rest is now veritical.
The down side to this is that the bed length is then shortened by the thickness of the headrest. If I tried to load a table into the back of the Blazer, I had to lift it up to go on top of the headrest so that I had enough length in the cargo area.
In the case of the Av, I could see this being a problem when loading 4x8 sheet goods. To load it, you would have to push the sheet in most of the way, then go in the back doors and lift it over the headrests, probably from both sides at once. Then, by the time you have several sheets loaded, that could be a lot of weight on those headrests, not to mention that the load would not be level.
I can see this being a great idea 95% of the time, but there just has to be a better solution that will work 100% of the time.
How about a notch taken out of the top of the rear seat backs so that the headrest can be pushed down one more inch? Or simply make the head rest an inch shorter?