MarkB:
I would think that Rivnuts and the bedrail fasteners are the way to go. If the Selby bedrail fasteners are the ones I am thinking of, where the plates and hooks are about 5mm thick and the hooks are peened through the back, then the flanges of the Rivnuts will allow the peened back of the hooks a void to fall in to, and thus allowing the plate of the bedrail fastener to be right on top of the flange of the Rivnut, giving a much more solid attachment. You could add a washer or two underneath the hook plate if the void space is not enough to accommodate the peened metal. I might also use Loktite on the threads, but that is a personal preference. I also wonder whether epoxy might help in any way
I would feel better about this arrangement, metal on metal with threaded fastening, than anything else that I could think of. I don't know how many you have to do, but for the task at hand, given the possibility of numerous assembly/disassembly cycles, it is probably as quick, durable, and inexpensive a method as you will find.
The shortcoming would be the shear force on the bolts; you might consider putting another Rivnut just below where the hook plate on the steel post would end and put a hex head bolt that would serve as a stop for the downward force on the plate, acting in a manner to counter the weight put on the unit.
Hope this helps. Good luck.
TonyF