Has anybody retrofitted the 2010 reversing camera kit to an earlier A3

You could buy a bundle from Kufatec: Rear View Camera - "Universal" Bundle - Audi/VW-

Speak to Nige (NHN); he'll know what's involved.

Nigel will sort you out, i believe that amar has fitted one too

I have a Reversing camera on my pioneer unit, which isn't too different in terms of installation from the RNSE.
It's basically the camera, the holder and a bracket to get it all fitted into the boot lid. The camera connects to the canbus system and then is coded to the car so it knows to look for the camera when reverse is selected.
There is another way of using a multimedia connector do that you can plug in the aftermarket cameras into the RNSE which is a longer way of doing it but cheaper