Brightness control is implemented in the AN75779 project under processing unit requests"UVCHandleProcessingUnitRqts". These capabilities may or may not be supported by the image sensor, please check. In any case, the firmware implementation on the USB side of the controls for these capabilities remains the same. However, the image sensor implementation would differ. Therefore, only placeholder functions are provided that implement the USB side of these controls.
You can check the same by using your UVC host application and sending a brightness change command. You can put a UART debug print in the UVCHandleProcessingUnitRqts to see if this is getting called when you send a brightness command. Please do check with multiple host application on Linux and Windows (this will help to identify if the host application itself if not sending these command properly).