I have seen this done as a mechanical failsafe on the throttle for PPM receivers.
THe trick is to have a low mass servo (coreless) and no binding so that a low tention spring can be used.
Putting a 8 oz load on a 42 oz servo is a low current drain solution.
ORIGINAL: MinnFlyer
I saw that setup too, and I can't help thinking, "Why would you want to put a constant stress/Battery drain on your throttle servo?"