VW called this afternoon to say that they won't replace the remaining wheels, as 'the corrosion could have been caused by force applied to the wheelnuts'. (Hmm, despite this being a foreseeable situation for wheels, then?). They said that it was the dealer's decision, and although in their view, if the dealer was changing the 2 unkerbed wheels for corrosion away from the rim (which they are), they should be changing all 4. But they can't tell the dealer to do that (oh, really?). And that wheels aren't a VW warranty item.
Kinda (actually hugely) annoyed. Will have to visit the dealer again to have a full & frank conversation about this.