I'm not sure about this, but I think that even on automatic transmission cars, cruise control is not the same as locking the accelerator. Cruise control maintains a constant speed, meaning that it'll accelerate more when the car's going up on slopes, and decelerate when the car's going down. If the game respects that, you'll have something a bit different from cruise control.
Still, if you find no other solution, you might always just put a weight on top of the accelerator, and something beneath it preventing it to go down