Rear loop strength

I’ve seen several Commandos with a drooping rear loop, so yes they do bend under load.

Back in the day, rear racks for the Commando had a brace from the foot peg to the rack on both sides. I would suggest that this would be better than clamping to the loop

The rear loop is known for sagging under heavy loads. Later frames extended the gussets to reduce the problem.

I'd think it unlikely that you'll have much of a problem - rider and pillion sit further forward on a Roadster. And you can always bend the rear loop back if it does sag. More of a concern would be hanging a lot of weight high and off the back of a Commando can really screw up the handling. Best just to try it.

It was quite common for stays to be fitted in conjuntion with Craven rack and panniers.

In Norman Whites book, Norman deals with a drooping loop. The loop is cut off and steel rod is inserted. The loop is then passed over the rod welded back together. This gives extra support to the frame. Andover Norton sell new rear loops if required.

Supporting rods down to Z plates is an absolute must imo 

It's not just the all up weight to think about,it's the weight times several times with each bump you hit