I don't find butt-steering a bike to be terribly difficult. In fact the heavier the bike the easier it is....the weight acts as a damper. That whole law of inertia thing.
I guess the challenge is navigating traffic for 500 miles using only sluggish butt motions. Doesn't seem like a shining advertisement for rider safety.
Usually the only time I see people ride no-hands is to make use of both hands to flip someone off....apparently when one just isn't enough.