Answered: Using a VEX 7.2V battery with the Cortex Joystick

Is it competition legal to use the VEX Joystick with a 7.2V VEX battery via the use of dummy AAA batteries inside the controller? (IE no cutting, modifying of the actual electronics/casing of the controller)

Technically this would be legal since there are no restrictions on the type of batteries used in either the VEX Transmitter or VEXnet Joystick. However, any team choosing this approach would be doing so at their own risk. Also, as stated in your question, you may not modify the VEXnet Joystick in any fashion.