I think it might be a bit broken. Here's what I did, on codereview.appspot.com:
1) Upload a new review (http://codereview.appspot.com/6242), click the
"edit issue" link. Typed john.ab...@gmail.com as the reviewer.
2) Clicked "publish+mail comments". It shows JohnA (his nickname on
Rietveld) in the reviewers field.
3) When I submitted that form, I got an error about there being no
such nickname. I had to type in his email address again.