I think you may have some misconceptions regarding the physics of motorcycles. You do not have to lean at all to turn, if you don't want to. Obviously the combination of you and your bike must lean, but you yourself can maintain a perfect upright posture if you'd like (and let the bike "do all the leaning"). You won't be able to turn as sharply, of course, but it's NOT the rider leaning their body that causes the bike/rider system to lean, it's purely a reaction to the countersteer you apply to the bars.
When I say "you do not have to lean at all to turn", I was not clear. What I mean is that you can look like the left photo, rather than the right photo: http://i.imgur.com/K4vTdDm.gif
I read your comment to imply that the rider's body lean is what initiates a turn, rather than their handlebar input. Or that the rider must actively lean (their body), rather than allowing the bike to lean them over.
Your other reply to jsprogrammer says that "it's pretty obvious that the passenger needs to be shifting its weight to achieve that." But that's precisely what I'm saying is wrong. No weight shift is required at all to do any street-riding turns. When the handlebars are steered, the bike will force you over. When a seated driver of the C1 turns the bars, the seat will move their body over, they don't have to do any work.
If you're on a racetrack, then yeah, shift your weight and eek out sharper turns, but you'll look silly shifting your weight around when riding on public streets or a highway.
My favorite link to paste on the subject: http://www.superbikeschool.com/machinery/no-bs-machine.php