I've had this feature on my 1997 BMW 540i, but I was not aware of it in the RS6.
If you turn the mirror adjusting knob as to adjust the passenger side mirror, if you put the car in 'Reverse', you will see that the mirror moves in such a manner to make the curb visible.
As I said, I've had this feature on the Bimmer for quite sometime, but never bothered to read the manual fully to see if it was available in the RS6. I always found it great to use, since I hate to curb wheels.
Anyone else aware of this feature?