I started sketching ideas for something similar. I have an old Schinn Twinn that I recently built up with a Shimano redband 8 speed hub. My idea is to remove the rear saddle+seatpost, stoker bars and pedals and fabricate some sort of metal tubing rack that would fit into the stoker handle bar clamp in the front and have a seatpost welded onto it to fit the stoker seat tube. Once I take measurements I'll make some proper drawings.
My first thought is the clearance issues from the spinning stoker cranks. I imagine I'll specify some bend in the tubular rack to allow the cranks to turn behind it.