Here is a thought. A hook like the ones to hang a bicycle in the garage, a chain, and a rope for each side of the bar. The chain holds the additional weight and attaches to one end of the hook. The hook goes over the bar of course, and then the rope goes from the top of the hook to the top of the power cage. Adjust the length of the rope so that when you get to the bottom position, the hooks will be pulled off of the bar. You might have to play with things or even use a mini-band to keep the hooks from re-engaging the bar on the way back up, but I’m thinking that this might work and you could throw all the stuff in your bag.
Anyway, that’s my mad scientist for the night.
Actually, this gives me a pretty simple idea. I think I will try to rig up some ropes with hooks to hold the extra weight on either side, so that the weights are on the floor at the bottom of the squat. Then my partner can quickly slip the hooks off, and I’ll do the concentric. Since the weights will be on the floor, there won’t be any uneven loading, and it will be simpler than ropes to the top of the rack. Thanks!