Ok i've looked into this before, and the problem i found was that because the FWD's have the disk and hub integrated together you simply cant get larger disks to fit it...
Now while searching for rear antiroll bars i noticed something interesting:
The rear axle on a 95-97 A6 (and some audi 100's amongst others) uses the same stub axle as the FWD A4, however instead of a one piece disk/hub assy, they use a small hub which takes the taper bearings just like the A4, then the disk sits over that as it does on a quattro A4.
Obviously it would require some measuring etc to see if its possible to get everything to line up, clearly the hub will fit the A4 as the stub axle is the same, however i dont know if the usual A4 quattro upgrades will work, as i dont know if the offset of the hub/disk assy would be the same as the quattro setup. (and the A6 disks are the same size as the A4 ones so no upgrade to be had there)
I might try to get a hold of these hubs and test it out, but if anyone else decides to try it in the mean time, i'd like to hear your results!