Is it appropriate to contact a journal editor when they have already decided to accept a paper you refereed?
As I have now heard back from the associate editor, I can give an answer to my own question.

The editor was quite appreciative of my note.  Here is what he wrote back to me:


  Thank you for your note. I am sorry that we did not realize that the authors have not satisfactorily answered to the criticism in your second report. If you can send us quickly a note that we can forward to the authors, we will have them take it into account before proceeding with publication.


I quickly put together a detailed explanation of what was still wrong with the manuscript and sent it back to the editor.  I'm not sure precisely how things will proceed from here, but I am gratified that I apparently did the right thing.