Don't forget that the throttle barrel has an arm on it just like the servo. To get the type of "linear" response that you're talking about (one degree of servo movement = one degree of throttle movement), you just need to set up your existing linkage correctly. If the arms are parallel you will get very nearly "linear" response. The helicopter guys do this all the time. Using a linear output servo (no arm) actually gives you an exponential control situation: you will get more "response" at the extremes (idle and full-throttle). Linear servos were used quite extensively before we had expo adjustments in the transmitter.

John Pavlick
Team Black Magic