if you used an engineering polymer, like the stuff they make gears from (maybe an acetal like Delrin) it would be plenty strong enough, but cost would be higher. They have replaced lots of metal parts with that stuff. A few prototypes and some testing would reveal the most suitable, lowest cost material.
One good thing about using plastic is no danger of stripping the plug threads... and by it's soft nature plastic is already "padded".
But the rope is more of a universal tool.. a tool that needs a particular thread is limited in application.
Maybe there's a way to temporarily secure the piston stop without using the plug's threads.. some kinda expanding mechanism...