Wblakeney,
In lieu of an external receiver battery you might try Scorpion Backup Guard. This device functions as a battery backup in case the BEC fails for some reason. If the BEC or any other power source fails then the Guard kicks in similar to emergency lighting unit found in schools, hospitals, etc. The unit supplies regulated 5 Volt power with a 10 Amp peak from a self-contained 2S/500 mAh Lipo to your receiver and servos for continued flight but without the E-motor. It plugs into a spare receiver port or can be connected to the receiver via a Y harness. It is about the size of a package of stick chewing gum and weighs 1.2 oz or 38 g. The price is about $ 25 USD and available from gravesrc.com. I use it on all my e-sailplanes and EDFs to prevent a flyaway or crash.
Rgds,
Art ARRO