Concerning an actual project, i'm also interested in that topic. Myself I think, that probably steaming/bending an already heat treated piece of wood will not be as effective as steaming a non heat treated piece - the given and heat treated bend is already fixed by the first heat treating more or less. So i'm expecting less effect from the second steaming and a greater tendency to return in the former shape. Also a second heat treating will be necessary. Let's try! Just my 2 (theoretical) cents