If you lose power, the fail safe (FS) won't help you, only the TRS (throttle return spring) will. When power to the receiver is lost, the FS won't be able to send your servos home.
What servo are you using for t/b? Curious to know which one it is so I don't buy it.
An electronic fail safe can do 2 things, depending on how it's made. It will go to a preset position when it loses signal from the transmitter and/or when the voltage of the receiver pack drops down to a dangerous level, it will go the previously set position.
If your FS is built into the receiver, then it will typically go wherever the servos were when you bind the receiver to the transmitter. I set mine when the wheels are steering straight and the throttle is at idle. I never cared to set it to full brake, if it kicks in mid-flight, I wouldn't want the tires to lockup causing it to flip and sending it to it's death.
If your FS is external, then they usually have a button or something on them that you press when you have your servos where you want them. Then that is where it will go when it loses signal or power power drops.
I don't know of any built in receiver fail safe's that monitor voltage and engage when needed. I've only had spektrum, and all of those only help if signal is lost. The old micro ofna failsafes I used to use when I ran AM/FM radios would do both, monitor signal and voltage.
It's strange that your servo locks when power is lost to the receiver. Some are hard to move off power and generally is a bad idea to try due to the strain you put on the gears. But when your vehicle is screaming away from you towards a lake or a person out of control, hurting a servo is the least of your concern.
Remember though, I don't believe there is a fail safe in existence that will do it's job when power is lost to it or the receiver. It will only work when signal is lost or power is low.