Ahh. I like where you're going with this!
So for the throttle portion you could indeed use a digital pot though this would mean solder re-work on the Z-controller circuit board. I'm also imagining an opto-isolator might be involved to provide electrical isolation between the Arduino power supply and the Z-controller power supply. That is, I'd have to do some homework to see if the electrical DC "ground" of the Z-controller is the same as an external Arduino.
Likewise, for the bell, whistle, and direction buttons, it might be as "easy" as an opto-coupler (with transistor output) since I'd think the Z-controller buttons are simply momentary pull-down switches. So these could be controlled by any Arduino digital output.
Your method would still be just a few bucks in parts without the mechanical nuisance of mating the servos to the physical buttons/knobs of the Z-controller.
While rolling-your-own triac circuit seems simple in theory, there are i's to dot and t's to cross that I don't recommend to the weekend DIY'er. So there are (at least) two inexpensive approaches to mating an Arduino to an off-the-shelf triac controller.