At first I was stuck trying to figure out out to "insert" the Guest Additions CD image in Windows because I presumed it was a separate download that I would have to mount or somehow attach to the virtual CD drive. But just going through the Mac VirtualBox Devices menu and picking "Insert Guest Additions CD Image..." seemed to do the trick. Nothing to mount, nothing to "insert".
Elsewhere I found that the Guest Additions update was part of the update package, so I guess the new VB found the new GA CD automatically when Windows went looking. I wish I had known that to start.
Also, it appears that when I installed the Guest Additions on my Linked Base machine, it propagated to the other machines that were based on it. Sweet. Only one installation for multiple "machines".
I still haven't found that documented, but it appears to be the case (probably I'm not looking for the right explanation terms because I don't already know the explanation). How that works should probably be a different thread.