[Al Skierkiewicz]

Martin,
I am going to go out on a limb and state that fasteners are expected for attaching the cloth to the backing material. Be they staples, tacks, etc. (or barbed wire fencing nails) as long as the other rules are followed, they will be acceptable. The aluminum angle is "optional" so it cannot be the only attachment that the GDC expects to see.
As to changes in humidity, I can only see that bumpers from a desert would swell with humidity making them more tight. The plywood, and to a very small extent, the Cordura fabric and the noodles, are all going to absorb some moisture. (Assuming you are not buying the Cordura Stretch Fabric)
All of this being said, everyone that knows me knows that sagging, ugly bumpers do not impress me. I am a TV engineer and it just doesn't look good for TV.
Dave, Inspectors will only ask to disassemble bumpers if they think there are illegal hard parts used int eh construction. And yes, we have seen steel and brass round bar stock and threaded rod inside the noodles, steel plate behind the noodles, and lead melted into holes in the plywood. If you can think of it, we have seen it.