Why not ride on the seat?
It has been suggested.
I don’t think its worth all the time and money to make, you will never actually practically use it for anything other than the casual “hey, look what my uni can do!”. It would just add on more things that could break.
I think this idea is up there with the hydrolic powered swinging grindplate that allows you to grind on the tire 
Lol, that gave me a pretty funky vision of standing on the seat, in seat drag position, following the wheel down the hill. weird but cool Then I figured out what you meant, and I tend to agree 
I actually like that idea:o