In the US, you no longer need a headache rack or bulkhead on trailer, in Canada they still require it.
As you can see above, it is only for vehicles with the load contacting the headboard. A headache rack or headboard is not mandated to be on the truck.
(b) Height and width. (1) The front end structure must extend either to a height of 4 feet above the floor of the vehicle or to a height at which it blocks forward movement of any item or article of cargo being carried on the vehicle, whichever is lower. (2) The front end structure must have a width which is at least equal to the width of the vehicle or which blocks forward movement of any article of cargo being transported on the vehicle, whichever is narrower. "arcticle of cargo being carried"
A headache rack can never be used for cargo securement as it's not on the trailer and freight should never touch it. Your quote is for cargo securement only.
Keep your chains, binders and straps on the passenger floor, take the passenger seat out for more room. I saw a dirty 1990's freightliner in the flyin-J in Phoenix, there was an old Mexican working on his truck with the door open (tools laying all over the floorboard). His passenger seat was gone and he had a pile of chains and binders on the floor. I said "what happens if you roll your truck over"? He answered "I never thought of that"... Heres your sign.
The reg you quoted I believe pertains to vans, refeers, and straight trucks with either flatbeds or boxes mounted to them. Nowhere in that reg you quoted is there anything that applies to open deck trailers or the tractors that pull them. Keep digging.
Could you imagine a load of steel pipe pushed up against the headache rack and you make your first turn :0
