I'd love to see an explanation on how to do just that...I know the concept behind a separate BEC (and using something like a 4 AA pack for servo controls; I used to run something like that with my old Tamiya RC cars) but I've never wired in a BEC to override one in the circuit. I guess I've always figured that the less weight, the better, when it comes to aircraft...
Definitely less weight is better, but it's balanced by power demands. I wouldn't bother with a separate BEC unless there was a specific, greater demand than what any built-in BEC/ESC combo could provide. I have two planes that have a separate BEC from the ESC. One is a micro sized FPV plane (a Micro Skyhunter) where I use a separate BEC (actually a buck step down converter) to power a 5V FPV all in one. I prefer my FPV gear not to be powered by the same BEC powering the servos in this case. In this case, the separate BEC/FPV power is coming off the battery packs balance connector.
On another, larger plane, with 6 servos (2 ailerons, 2 flaps, 2 for elevator & rudder) I have another separate BEC to power the ailerons and flap servos, and use the built in ESC's BEC to power the elevator, rudder and receiver. The separate BEC powers the ailerons and flaps, and I chose this because this plane (a sailplane) requires me to put the wings together and connect the servos as part of setup. For this, I made a custom wiring harness that's always connected between my receiver, the BEC, and the pack battery leads in the fuselage. When I connect the wing servos, I connect it to this harness. There's a common ground between the RX, BEC, and battery. Only the signal pins/wires are connected between the servos and the RX. The power positive/red wires go from the BEC power output into the 4 servo female connectors on the harness.
So, this harness has four male JR/servo connectors with just the ground and signal wires connected. Those connectors go to the RX. The ground wires are all soldered together, and in turn direct soldered to the BEC's ground, on the same side as the 5V output. On the other "end" of the harness are 4 JR female servo connectors that my servos connect to. The 5V power output of the separate BEC is directly soldered by wires/pins to the middle pin of all four female connectors, and the signal wires from the RX end go to the appropriate one. The servo connectors have labels on them for "LF" or "LA" for left flap, or left aileron and RF, RA for the other wing. Left and Right based on nose out setup when hooking things up. It's a lot of wires, but actually makes it easier for me to hook things up vs connecting and disconnecting the servo leads to the RX.
I don't have a good photo due to the cramped nature of this fuselage and lack of photos before I put everything in, but hopefully my description above is clear.