Why use a BEC to get 2S voltage from a 2S battery?
First, if the receiver is high voltage capable, pull and insulate the positive wire (typically red) from the ESC to receiver plug. If not high voltage, do the same with the servo to the receiver. (leaving the ESC to power the receiver only)
Then replace that power, either with a splice from the positive side of the battery to esc wire. Or another possibility that would provide switching, is to splice off of the switched side of the ESC switch. I used to do this with the old boosted KO servos that also used direct battery voltage.
This new wire goes either to the receiver, or to the servo, depending on the first step.
Oh, and remember not to toss in a bigger battery, like a 3S, for some speed run fun. If that is a possibility, then the BEC starts to make a whole lot of sense.
(no comment on why I mention this
)