I think Amel did it on purpose. Hot salt water will corrode metal pretty fast and if you go with metal it will become constant maintenance/replacement issue.
after all it served you for more than 20 years 🙂 without much maintenance on your side.
I would look into either doing the same, or look at what Amel did later with 54s as they added a flap to prevent back flow (per my understanding).
Another option to look into things like marelon or other non corrosive seacocks, that can deal with heat.
(in search of one)