This is true to a point, moving the center of gravity horizontally back would put a longer moment arm on the force acting to bend the seatpost. But can you imagine carrying 20 kg on your back and trying to ride the bicycle? I'd be more concerned with the vertical rise in the CG and the concomitant loss of stability. More like this is a worst case scenario, and 20 kg is a lot of weight to be dragging around no matter where you put it. Better to go lightweight and pare down to the essentials I would imagine.

Re the Nitto racks, can find the rear racks via google, but they do come dear at $180 US.

I would not do it, not even 500 mtr but when you see schools in this area on a over night trip almost 100% of the kids (approx 15 yrs?) is riding fairly new, not so cheap MTB`s, no racks and huge backpacks. Not the same problem with seatpost (since they are shorter) but who want to ride like that? Some of the bikes are "bought to last a few years) so they are too big. Lots of weight on the arms. Nothing suprises me anymore. It is not about money but about knowledge.