The receiver typically distributes the power regulated by the BEC to the servos by means of having the 5v lines of the servo headers ganged together - you can check the continuity from any middle pin to any other middle pin on the RX, they're all connected.
You can remove the middle (power) pin of the servo connectors and tie them to any other power source you'd like and they'll work fine, so long as both power supplies (the BEC powering the RX and the servo's power source) share the same ground. This is what I prefer to do in twin engine craft when both ESCs have integrated BECs: Since just tying the BECs' outputs together isn't recommended (the regulators could end up "fighting" each other, leading to failure), I plug some servos into the RX directly (off the first BEC) and some into the second BEC by removing their power pins from the RX and tying them together separately. This spreads the servo load across the two BECs without directly tying them together.