what you're suggesting is an option.

it really depends on the measurements of the coaming (its width) relative to the size of your butt and accomodating padding and your seat option and the back-band attachment set-up.

for example, a night heron coaming, if laid out per plan, is only 15 3/4 inches wide which is relatively narrow, but for me, wide enough to get my butt through the cockpit opening, but then it opens up and accomodates a seat that is closer to 17 1/2 inches below the coaming (when your butt is against the seat it is wider than when suspended and going through the cockpit opening). so this arrangement works (at least for me)

so if you took the verticals for the coaming and also used them for the hip braces, in my case, you would find it too tight and its not going to work and allow for any padding on the hip brace and the back-band, which attaches to the hip brace, would also interfere with my butt if you had the coaming and the hip brace on the same plane.

that said, i have seen some relatively wide cockpits use the approach of the coaming and hip brace all lined up.