I know a few guys with spoilers on their A3's and they all still get pretty dirty. I presume you are thinking about a roof mounted lip.
Vortex generators look cool and help a little with aerodynamics but these still won't stop the rear getting covered in muck. I actually think most of the much comes from the sides and from below and get sucked on the back as the car moves along.
Thanks guys.
I also think the spray and dirt comes from under the car and was thinking a spoiler below the rear window would maybe do the trick. There are an awful lot of A3s out there and if I had the means I might just have a go at sorting a spoiler myslef.