Most newer servos handle 6.0 volts well and should live on unregulated 6.6v LiFe voltage just like your 645mg's do (I've been flying that exact same setup on 5 cell NiMH since summer '08 and have had no trouble). Older 4.8 volt servos may not be happy on 6.6 though? Rather than messing around adding stuff/complexity trying to drop the receiver buss voltage down so you could use an old servo, I'd just update to a 6 volt servo. That's me though...

Have you had one of the failed servos apart to check the gears? What are you using for a pushrod between the carb and the servo? Is it possible it's heavy enough or there's enough strain on it to be wiping out the servo gears?