With regards to a throttle return system, this has been done for years by racers worldwide. And it works. It doesnt matter how strong your servo is, once it looses power the strength goes out of it and the band/ spring shuts off the power almost instantly. Only thing it doesnt do is apply brakes.
As for what to use, yes you can get springs designed for this job, but if you have a sister/ girlfriend/ wife, then youll have access to those elasticated hair bobbles they use. These are a better alternative to elastic bands as the bare elastic can degrade over time. Plus if you want style, you can get all different colours...
Thanks
Chris