Someone needs to weigh these. How much weight can a little piston have? Xray has offered arm weights for years, to calm the cars down. But I don't think adding weight up high on the car is such a good idea.
Generally, shock pistons are made of a soft alloy. Brass should do:
The piston ring (or packing) is teflon coated brass:
Maybe the piston has to be metal. All the oring pistons are metal. Maybe plastic will fail around the little oring cuts.