hiya chaps!
an electronic failsafe works only when there is frequency jamming or interference.
i suggest, and please consider this with capital letters, RUBBER BANDS!
use rubber bands to make sure the throttle closes in the event of battery failure.
further to this, please consider using a digital servo for throttle as digital servos tend be less likely to bind off power (personaly observation), analog servos tend to be stiffer to turn and may require an insane number of rubber bands.
just hoop the rubber bands around the throttle ball stud and secure to the far side of the throttle. making sure that there is enuff tension to close the throttle off power, but not too much such that your servo is fighting the rubber bands constantly.... that might burn your servo or strip gears
i hope this helps.
-alexander
p.s. also get a electronic failsafe so your throttle won't be locked at full bore during freq interference