Ok I've been looking in to this for a little while originally for my 2.8Q SC but now for my 1.8TQ.
I figured it could be done but could not find anyone who knew how but I had a good idea of what was needed.
I will post up what I know right now but I'm hopping to nail down the details from someone that as done this and also offers it as an option.
There are minor modifications that will need to be done, but it's simple
Parts needed:
6 speed trans
linkage & stabilizer bars
center shaft
rear dif
left & right trans mounts & brackets(left one will need fabrication)
either A4 rear dif flanges, or S4 1/2 shafts
To eliminate the fabrication to the left trans mount, you could either install an S4 front subframe, or weld on a bracket to the A4 subframe to fit with the S4 left trans mount(which I intend to do soon - better IMO).
3" downpipe will require modification to the right trans mount bracket. Shifter area in the center console is a little tight for 4 gates, but does just fine.