I think that we posted messages last year in the talk pages of users without an email address. Although I can't find evidence of that in the archives, so maybe we only talked about doing so.
Anyway, I developed a tool for listing WLM participants without an e-mail: http://toolserver.org/~platonides/wlm/nomail.php
It lists users without an authenticated e-mail. I recommend that each local organization team prepares a boilerplate in their local language and posts to the talk pages of the users that participated with photos of their country (if they decide sending such messages).
If when you are going to notify a user you notice that you have the "E-mail this user" link on their talk page, then they *do* have an email set, it was a false positive from my tool (I saw some instances, either due to replication lag or the toolserver copy is desynchronized).
There will also be some false negatives, for people that do have an email address set in their preferences but have explicitely disabled receiving emails from other users (which is what we really care about). Hopefully, there will be few of those.
Regards