Those pannier supports are made out of the same heavy duty tubing as the cyclerack itself and they way they lock on to the rack, I doubt you could ever bend them carrying any reasonable (or unreasonable) load on them. They are made to attach aux gas tanks to and can easily carry that amount of weight. The bungee cord only serves to keep them from swinging out sideways and to make sure they stay locked on to the rack. You could tie them with a length of rope or cord if you want something more secure, or easily attach a metal bar connecting them. If you weld a bar between them, you will never get them off unless you cut the bar or unweld it. Better if you want something "permanent" to make it so you can screw or bolt it it together.
I like the idea but never bought them because I thought at the time they were too expensive,