According to this blog post the new messaging system should be accessible via the API (fql/graph) for non developer account as of end of November. It still gives OAuthException with message “You must be a developer of the application”.
I was unable to find any updates on this issue from Facebook or other sources. Anyone knows what is going on with this and when will it be accessible?
There was a bug opened on the subject, anyone who has interest in this might consider promoting it.
Going to answer my own question, according to the response on the bug seems like the official answer is:
“We said it would be published but it wasn’t, so just wait until we say it will be published again”.