All the Box2D documentation says to use the automatic gear joint and not to mess with simulating gears this way ...so it had to be done! A little trigonometry and fidgeting with sizes produced this simple planetary set:
I quickly modified my sprocket construction code to make the annulus and spent a while scratching my head before remembering that the shape vertices have to be defined clockwise. Once they were, everything was interacting. But in all honesty, I have to nudge the annulus to get it going no matter how much torque I apply to the sun gear. It would be great to understand why if you know. And it would be really cool to develop an "eccentric drive" or two like these: