What is a P clamp?
The main weight, as you know, is on the stuts, and the rest is just accelerating and decelerating, roughly. I can think of any of a billion dead-nuts ways of attaching a rack that is missing the upper braze-ons. In fact my bike had the correct attachment points, and it was really nasty getitng them to fit. I can imagine any number of home made solutions that would have been more elegant, which reminds me.... In fact if you look at the 10 dollar gizmo that OMM is selling for those without BOs or Cantis, it is hardly a masterpiece of engineering. But crossectionally it probably has more on the go than most rack bolts.