The father-in-law has an XC70 which has a folding steel grille which is attached to the ceiling anchor points and folds down behind the rear headrests. It's great, as most of the time it is out of the way, but on the odd occasion he comes to pickup our dog, it provides an extra bit of safety to stop the dog coming forward in the event of an accident.
On the V60 we have a mesh load protector that mounts on the back of the seats and pulls up, and I had a similar one on my old Jag X-type.
I've got a folding load protector net that came with my car, but it's not as rigid as other ones.
The dog goes in the car maybe once or twice a month, so most of the time, I don't want a full fixed grille in the back, as it gets in the rear view, and prevents 3rd row being pulled up. One that folded from the ceiling ala XC70 would be really good. Volvo don't seem to do one for the XC90, does anyone know of any aftermarket manufacturers of such?