Well, as far as the cost goes, the microchip in the servo seems to be the cheapest thing inside.
Maybe the chip cost in total 50 cents and the rest is Motor+Gears+Box etc, all multiplied by 300%
proft margin at least ... well those guys got to eat something ... You can trust me a "bit" more since
I am in that business about 30+ years or more.
If it was a zenner diode, ti would just limit the voltage to 6V and the only trouble would be higher
curret drain trough the Zenner diode till the battery voltage drops to 6.0 V. That is why I suggested
to be measured the current consumption in a Laboratory.
Most likely the "protection" alike behaviour comes from the Feedback Electronic Circuit which monitors
the position of the Potentiometer attached to the servo arm. That potetniometer "tells" to the circuit that
the rotation angle has been achieved or not, and from there comes the signal to stop or continue
running the el.motor inside. That feedback system is also designed to operate at Wide Range
of Battery Voltages and Temperatures and if one exceed that Battery range, then the system
gets "dizzy" and wander what the "H..." is going on now.
Voltage stabilizer in the Servo is also a bit trouble to do because that takes space for heat sink and
more importantly is would affect the Speed. Since almost all servos with price difference of 10x the
speed is usually increasing with the Voltage increase from 4.8 to 6.0 volts, that shows that most
likely inside there isn't voltage stabilizer.
That is why I found more efficient to add it for my(!) needs outside. Below is the Electric Circuit
one can use.
But make sure that you have not less than 7 cells in order to allow the voltage drop over the
regualtor, which is about 2V. Of course, as mentioned, I needed this in order to be able to
have exactly(!) 6 volts and also use NiMH Batteries in temperature range -25 to +40 Deg. C.
That is a bit overkill indeed and costs some time to solder all those wires. No problem if your
side hobby (or profession) is electronics.