Ok anyone with park assist sensors on front s3 grille, this may be useful for you if you plan to fit the rs3 grille.
1. Firstly I purchased 2 of these sensor brackets of eBay. They are for BMW but honestly they are the only ones I could come across and they do the job with some cable ties as you will see below and without using the 3M tape that comes with it.
eBay product link
http://bit.ly/1s6vEKe
2. Here is a few test fit photos with using black cable ties to hold the brackets into place. The black cable ties camo nicely on the front of the mesh grille.
3. After putting the bumper on temporarily with the 3 screws under the hood, I took it for a test drive to ensure park assist works fine, and all was good.
Here's some pics with final stages of the fit:
I used black tape to black out the bracket, but I suggest spraying it the same colour as your grille rather than doing this. I will attempt to color/spray the exposed tape gloss black another time without removing the bumper.
As the sensors don't lock in with the BMW brackets all that well, I used a couple other cable ties to ensure they are locked in the bracket and won't fall out.
You may need to cut the foam for your crash bar to make a clearance for the sensor brackets. You don't want any force on the harness wires so I did this to avoid that from occurring.
4. Final fit, pleased with the result. As mentioned above ill colour spray the bracket a nice gloss black to blend in more with the grille as you can see colour difference slightly. I will also find out if I can color the sensor from silver to black with a small coat of spray paint.
Productive DIY day.
Hope you find this helpful if you go with this procedure or find other ways to mount the brackets.