I was thinking of something like that for my seats, too. I have a pretty substantial seatback frame that I could easily attach something to on the side so what I need is less involved. The hard part would be matching the vinyl color. I would be looking for a corresponding rest on the door panel, too, just to balance things off.
If I keep adding stuff like this to a "Speedster", then some day it will become a "Cabriolet"!