I haven't experimented much, but just from the physics I think the instability comes mostly from weight that is off-axis. Someplace recently I saw a picture of a bike that had a child seat mounted on the handlebars. Of course is was a cruiser/comfort type bike with an upright posture for the cyclist. I imagine it would work ok. Even though the child's weight is high, it is centered on the steering axis so it isn't pulling the steering off center.

http://images.businessweek.com/ss/09...st_bikes/7.htm