The way I see it, there's no need to do anything with the stem. An Audi A4, B6 type, will even fit directly to the boss on the A2 windscreen. Looking at some auto-dimming mirrors on Ebay shows that to be the case. There appears to be two types of boss glued to Audi's and hence there can be two types of mirror. The Audi A2 boss and the Audi A4/version B6 use a smaller clamping/locating plate which has a pair of vertical claws that align with the boss. Turning the mirror clockwise then locks it in place on the boss. Now, on to the other type .... its plate is a larger diameter and has three claws. Obviously the windscreen boss would be different, too. Both of those clamping plates are removable. The Audi A2 is held in place with a central rivet that has to be drilled-out to remove the plate. The other, larger version is secured with two screws, so removal is very easy. Now because the other type of auto-dimming mirror has a larger diameter plate, the A2 plate will easily fit in its place. The modified mirror will then locate directly to the A2's window boss. That simple metal plate is all that differentiates a wide range of these Audi mirrors. There is absolutely no need to do any other modifications .... no drilling, Dremmeling, etc. Just changing the plate should give a usable mirror.
Furthermore, there is then a much wider choice of prospective mirrors from which to choose. There ARE a few that are unsuitable, though. A careful look at them should reveal their suitability ..... I have seen some as cheap as £20-£30 !
I will give more detail and photos as I progress with this latest project.
David