So I finally came around to testing this setup. A 9g hobby servo, with an additional cable hooked up to the middle pin of its pot, and connected to an analog pin on the arduino. Voila, now we can sense the position of the servo. Kinda.
Turns out that even with a lot of small capacitors all over the place, and software smoothing of the signal, I still can't get more precise than about 2°. Which is actually quite nice already if the only thing you need is to tell if your servo "got there" already.
But I wanted much more. In particular, I wanted to be able to tell the magnitude and direction of the force working against the servo. Then I would be able to tell if the leg is supporting the robot's weight at the moment (and hence wasn't put in the abyss beyond the table), or even implement compliant motion, like in this video:
Alas, turns out that it's not so simple. The pot measures the servo's position quite well when it's unloaded. but as soon as you put any load on the servo, the reading is going up -- and that's no matter in what direction you apply the load. That's not so good, and I think it may have somethig to do with voltage drop on a loaded servo. I will need more experiments.