You can fit a towbar to a RS4 B7
Got my B7 back in July and missed the ability to carry my mountain bike on the rear of the car. Last weekend i drove to wales and got so naffed off with wind noise from roof rack decided to see if a normal A4 Avant towbar would fit.
Simple answer is YES it fits.
From what i can gather is its illegal for me to tow a trailer as the RS4 was never type approved for towing. However this does not stop me from using the towbar for non towing applications ie towbar mounted bike carrier.
As you will see from the picks i did have to cut the towbar a tiny amount (well a fair bit underneath but only a small amount visable).
The towbar was from Towequipe along with the A4 specific Westfalia electrics which plug into the wiring loom. Took about 6 hrs to fit all in.
Attached are some pics.