A large electrolytic capacitor on the 12V side before a 5 volt regulator may provide enough hold up time to reset the servo position, experiment with the largest electrolytic you have, but keep in mind servo's have no holding torque without power so the control arm will float with any force applied to the output shaft. If the load on the output shaft of the servo is light enough a cap may work, again though it has to be on the 12V side of the regulator, as soon as the power is lost the voltage will quickly drop but the time it takes to drop from 12 to 5 volts when the regulator drops out could be enough. One possible problem though is as soon as the voltage drops bellow 5V the applied VCC to the 555 will change so this make cause the timer circuit to send a distended PWM to the servo, all it takes is one bad pulse to tweak a servo a lot.