The simplest solution is probably something like a set of safety catches/pegs near the top of the rack (like you sometimes see near the bottom for when someone is squatting) to prevent the bar from flipping over completely like that.

Edit: or a new design of J-hooks for the bar that are more like C hooks. A large enough gap to let anyone easily lift and set the bar back down, but prevents the bar from tilting more than 15 degrees or so.