If you want to make sure you leave a legacy that is maintainable, then it seems to me that the most easily re-used solution would be to have the hardware basically emulate a solenoid i.e. go to one of two positions based on whether an input is high or low. Anything involving driving a model RC servo directly would require a PWM interface which I think would be quite hard to drive for somebody without electronics experience to deal with. Is it possible to simply use a solenoid per target? Then your Arduino would have a transistor per solenoid and activate each output from your sketch. When the Arduino dies and you aren't around to fix it, they simply replace it with a bank of switches to trigger each solenoid. (This is also a handy fallback if the bells and whistles solution goes tits up at a crucial moment.)