hmm, maybe I'm wrong here but the idea of having a spring return that would over power a broken servo dont seem like a good idea...I mean if your linkage breaks or the servo arm it would save you but if the servo was damaged internally and the linkage was intact your spring would have to be strong enough to over power your faulty servo, such a spring would not allow the servo to work at all... further more I would think any additional spring on the linkage that was attached all the time ( like it would have to be) would put extra strain on your linkage and /or servo

I don't claim to be an expert by any means, this is just my thought on the idea.