Amazing small gear, indeed. I was working in 2003 with Stefan Gasparin (micro-mechanical genious, have a look on the Web). We designed a 1.3 grams servo, see
Documentation du GdServo 2003. A hall sensor and a 4mm ring magnet was used for the position (I still have severals if you have a similar application).
By that time, light rudder actuators was the big problems for ultra-light slow flyers. Article dans la revue Cervia, 2008