So, a little bit of rethinking what I actually need, and the solution became obvious. I don't want to know the position of the servo, I want to know the force that is affecting it (and, at the same time, the force that the servo is using to counter that). So, it doesn't make sense to add the additional wire to the pot. I want to know how hard the motor is working -- so the additional wire should go to one of the motor pins. From that a couple of simple experiments, and I have my "compliant motion", more or less:

Of course, the code is very crude, so it's easy to get oscillation or runaway, but in the end I will use it for robot's sense, so it will not feed itself...