If a failed bec caused an esc meltdown, you did something wrong like not pulling the + wire out of the esc to rx plug. $1 says that is what you did.

I've had one bec go bad and pump full battery power into my rx and servo. The servo eventually died, the rx was fine.

An 800mah lipo is going to power a servo under load for about an hour, maybe two. You'll have to worry about over discharging and making sure it is charged every time you go out. Not to mention is it quite a bit bigger than a bec.