Like you, I have no idea about this as I do not follow CHDK ports.
So, I first check the cameras platform_camera.h to see if it is mentioned there.
It is not but that file tells us the camera uses property-set 5.
Maybe it is a property set ?
In propset5.h I see PROPCASE_ORIENTATION_SENSOR and a search for that reveals it is mentioned in notes2.txt (for IXUS300):-
221 RotationAngle ROM:FF93EFF8 GetRotationAngleResult PROPCASE_ORIENTATION_SENSOR