I borrowed that picture from KZ, I own a sportsman classic myself, not a spree :)
Biggest problem I see, there's no real studs in the walls, there's probably a 1x3 in there supporting the floating bunk with a screw or two. And a couple wood 1x2 won't support a grown man. a frame made from 1" box section of steel (maaaaybe aluminum but I wouldn't be so sure) would support the weight, but where do you anchor it... if your KZ is like mine, there's nothing in the roof, or some chain or cable stays would work.
If it had to be done, to support the steel frame for the upper bunk, I'm thinking a 3/4" piece of veneer faced plywood would fill the entire area where the little headboard currently is, and the opposite end for the foot board. By entire area, I mean from under the frame for the bunk all the way down to resting on the storage bay / bunk below. Then, in the storage bay under that lower bunk, place a few pieces of 2x4 to transfer the load from the plywood down to the floor.
I wonder if KZ would sell you a few yards of that crazy pattern fabric, you could upholster the bracing ply with some thin foam and that fabric, making it look like headboard. it would look stock to anyone that hadn't seen a before picture.