the 919 doesnt have a sub frame like sport bikes the frame under the passenger runs all the way down the bike and acts like a back bone IIRC. You would have to cut the frame and fab a way to mount the sub frame. would be a cool project!
http://image.sportrider.com/f/8597000/146-0204-919-FRAME-zoom.jpg
http://image.sportrider.com/f/8597000/146-0204-919-FRAME-zoom.jpg