[andrewflnr]

I don't think users would go for that UX, so if you're committed to building this feature, I don't think you can go that route without basically accepting that no one will use it. I suspect MS wants this to be a lot more casual than the other features behind authentication now. (Like I told akkartik, I'm open to the argument that there's no way to implement it securely and with an acceptably easy UX, but I don't see people actually making that argument.) Maybe they'll prove me wrong by doing something like that, though.