If your receiver can handle 7.4v nominal you would be able to power it like that. Typically people buy a short servo extension, cut the red(+) wire and plug it in between the ESC and receiver. This stops the ESC from supplying power to the receiver and servo. Then plug your secondary power source in to the receiver. The batt channel would work fine. Again, this can only be done if the receiver supports that high a voltage.

There are wiring diagrams out there for supplying power directly to the servo while only keeping the signal wire of the servo connected to the receiver but i haven't done it.