"It was a "Blossom-Time" sales promotion offered only during April 1969 to certain Michigan dealers. Documented by 2 different magazine articles, it is still un-certain exactly how many were done."
I'd bet there is a market for aftermarket rumble seat conversions for modern sports cars. It should be fairly easy to implement; design hinges to allow the trunk to open forwards for normal use (with seat cushions removed, of course), or backwards for rumble-seatage.
I mean, who hasn't dreamed of a four seat Corvette?