Disabling USB power on Mega 2560 w/RAMPS

Starting a new thread, this one is 3 years old: http://forum.arduino.cc/index.php?topic=179013.0

I'd rather not make any mods to the 2560. My printer has an internal USB cable that runs from the panel mount on the frame to the 2560. Couldn't I just splice a relay into the the 5V wire in that USB cable so that when the external (RAMPS) power supply comes on, the relay closes and power is supplied to the USB port on the 2560?