Help with custom flight mode - roll for yaw control

Hello. I have a unique requirement for an Arducopter flight control mode and I need a point in the right direction to start modifying the firmware.

I’m not permitted to get into the details of it, but when I change to this new flight mode, I would like the yaw control to the motors using differential speeds to be essentially disabled, and instead I would like small amounts of roll, left and right to control the heading, based on the usual yaw stick controls. Imagine a long stick attached to the drone, and the end of the stick fixed in space so that rolling causes the drone to move in a circle with the end of the stick at the center.

Any suggestions for what code I need to start focusing on to accomplish this?