So you're thinking of applying the pop-up air grabber idea to the rear spoiler? I saw a new Jaguar the other day, the back spoiler retracted when he came to a stop, it looked neat. The only things I can think of in your instance is the way it goes past the trunk lip, you may have to modify the rain gutter to make sure the water still flows out, unless it never gets wet again. Also, you'd have to have a sturdy mechanism for the risers, there could be a couple hundred pounds of downforce at high speeds.
Nice sketching, by the way!