Castle HV ESCs are opto isolated on the throttle input, meaning they do not supply any power back to the connected FBL/Rx. If you want to use a Rx battery pack, then you have a few options:
1) Put high voltage (up to 8.4v) capable Rx, FBL, and servos on the helicopter and power them directly off a 2S lipo flight pack. Alternatively use a LiFe pack and components that support that voltage level (don't know it off the top).
2) If you don't want to run high voltage components, then use a voltage regulator between the Rx battery pack and the electronics to step down the voltage to a safe level (up to 6v usually).
3) If you are thinking option 2, then why not just use a BEC and power the electronics through the main flight pack? What protection does a separate Rx pack through a regulator provide that the main pack through a BEC regulator doesn't?
If you're wanting some peace of mind, consider using main packs into a BEC coupled with something like a scorpion backup guard or an optipower ultra-guard. The optipower has some very nice features that, I think, puts it a step or two ahead of the scorpion unit.
