Hey Rockaway,
Did you ever figure out a rear rack? I welded one up with the help of a friend and it's held up well on my FE550. We bolted a rotopax mount to it and I leave it on there all the time. While the rack works well, I plan to upgrade from my Giant Loop Mojavi saddlebag to the Mosko Moto Reckless 40 and I don't see the need/point of the rack anymore. I plan to move the rotopax to my Tacoma, remove the custom welded rack and just use the R40 set on the rear fender. Fuel will go in the MSR fuel bottles, strapped to the back/under side of the R40 harness.
I have also been trying to find a solution for an underseat fuel expansion tank, but like you and your search for the larger main tank, they are very hard to come by. Contemplating a custom solution.....
I'm located in SW PDX, basically Beaverton, just east of 217.....would be great to catch up with a local fellow Berg owner sometime.