Look for adjustable (or reversible) leaning posts, and you should find something. Pretty popular with offshore center-console guys.
Usually it's made as a hoop of tubing, with the adjustable part being a pin on the tubing that drops into one slot or another on a stationary plate.
Of course, now I can't find a picture of what I'm talking about, but here's something semi-similar: http://www.arrigonidesign.com/leanpost.htm
(halfway down the page: "Pedestal mounted swing back seat"), then picture a couple of stops that the seat frame locks into. (Really bad description; sorry.)