For several years I had a seat post rack. I put it on a Cannondale road bike to help carry stuff and it seemed like the perfect solution because the bike was not designed for a "normal" rear rack. I used the rack for about five years and one day while riding home the rack busted. It cracked near where it attached to the seat post, I assume the victim of vibration over the years. I suppose I could have overtaxed the thing, I do not remember if there was a recommended weight load.
I was actually lucky when it broke off because it bounced off my rear tire and went behind me. It could have ended up in my spokes, or could have caused trouble if a rider had been behind me.
Seat post racks are appealing, but when I saw this hack I was not quite as excited about the idea as I would have been if I had not had my rack break on me.
This is a clever idea, I just hope that 1) the passenger is light 2) the post has a heavy duty weld.