While the possibility of servo-motors still exists, and has already been demonstrated to work under a fairly simple control scheme, servo-motors are somewhat expensive ($8-10 each) and exhibit a great deal of mechanical noise.
An alternative actuator is the solenoid. While solenoids can range in cost, I have already found a source that will sell them at $1.50 each ($1 in bulk). By using rubber washers and bumpers, the noise could be reduced to a quiet thud.
The challenge, however, is in making them efficient. To get the linear force from a solenoid, the power supply must deliver a great deal of current, which can be overly taxing on our power system. However, the current does not need to be high over a long period of time. A capacitor can be charged up with relatively low current, then discharged to deliver a very high current for a short period of time, which may be just enough to get the solenoid to pull, after which it only needs a low current to hold itself in place.
My ongoing task is to build the driver circuit that will direct power in the more efficient way, which will allow us to use more of them, or dedicate a larger budget to other subsystems.