After seeing your pick im tempted to impliment that in my next bout of car trailer mods, atelast for the regularly towed vehicles. I like your idea. Quick, easy, simple, cant see how it would fail.
I think the goal tying a car down is to keep teh tyres planted on the deck. The body can do what it wants, it has suspension, use it, will give the car a softer ride after all.
Most of my attempts at tying the car down by the body have failed, and upon reconsidering the failures (all picked before they became masive thankfully!!) i think its the jumping up and down of the body on its suspension, even if you pull a car down on its suspension over a big bump it might jerk, and either move and loosen or worst case break a strap, so IMO just planting a cars tyres on the deck is the key.
Cheers.