I know my reversing lights are 'illegal', like any other lamp, they have defined regulations. That said, I understand the implications for their use.
Here's the intensity regulations: as such that they need only serve as visual warning that the vehicle is reversing. Minimum axial intensity is 80cd, maximum intensity 300cd at any point on or above horizontal, max intensity 600cd at any point between horizontal and 5° down, max intensity 8,000cd at any point below 5° down. Beyond that, there are specific test points a reverse lamp must meet:
Points at H and 5° down
V: 80cd min
10° right and left: 50cd min
30° right and left: 25cd min
45° right and left: 15cd min
Points at 5° up
V: 25cd min
10° right and left: 20cd min
45° right and left: 15cd min
Points at 10° up
V: 15cd min
10° right and left: 10cd min
Reversing lamps are definitely regulated as on-road lighting devices.