As far as I know, there's no way to do this. As you mentioned the switch isn't inline with the main voltage. You'd have to find the BEC circuit on the circuit board and wiring in your external BEC to the input voltage side of the circuit. I just ended up connecting the external bec to the battery and putting a switch inline from it's output. Making sure I had a switch rated for the appropriate amps. I know that in this configuration, the BEC is powered if the battery is plugged in, but I suspect the draw is really low and what I wanted was to make sure my receiver was off.