What about something like this his, paired with the padlock of your choice? They are coated and possibly could be tightly wound around the seat stem, using a aluminum carabiner on the two loops to secure it.
not sure this is thick of a gauge wire rope to do the trick but this may lead you down the right path or give you ideas.
https://www.amazon.com/Stainless-Col...95714854&psc=1