Originally Posted by Retro Grouch

Weight is one thing, leverage is something else. Check out the bikes that you see anyplace where a bunch of bikes are parked. My bet is you'll find at least one or two with a seatpost that's bent backward. That's with a conventional seat. If you cantelever the weight of another passenger on the seat behind the rider, the force exerted by their weight is going to be multiplyed. That sounds like a recipe for disaster to me.