David82:
Why can't you just use a a 12v battery, a pot, and a 12v motor? This is an Open-loop system. Because of that fact, servos aren't necessary even though they could be used. There should be a way to construct a simple, analog circuit that consists of not much more than a battery, pot, and dc motor but still allows for proportional forward and reverse movement of that motor.
You could do this - but it's going to take a few more parts. I'm envisioning something using the parts you mentioned, plus a window comparator or two (to sense voltage above or below a mid-point value, taken from the pot as a voltage divider), with those outputs feeding into a square-wave VCO or something (for PWM), and from there into one side or the other of an h-bridge.
In short, you would simply be redesigning what is inside a servo - with the feedback potentiometer being the "controller", the motor of the servo running open loop. In fact, I would look into the analog circuits used by early servos (talking stuff from the 1960s-1980s or so) as an idea (or you could just use one of the various chips used in servos today; they still require more than a few extra parts - a couple of transistors for a half-bridge, and a handful of diodes, caps, and resistors - maybe an inductor or two).
It'd be easier to just emulate it all with an ATTiny (but you'd still need an h-bridge)...