On my local machine, this still works fine; tested with both mediawiki trunk and wmf/1.20wmf6 (the version currently on enwiki)

Config looks fine as well.

On enwiki, entries are successfully being added to the job table.
Example of a test-entry:
job_id: 7302165
job_cmd: ArticleFeedbackv5MailerJob
job_namespace: 0
job_title: Golden-crowned_Sparrow
job_params: a:5:{s:9:"user_name";s:8:"Aft5hide";s:8:"user_url";s:42:"http://en.wikipedia.org/wiki/User:Aft5hide";s:9:"page_name";s:22:"Golden-crowned Sparrow";s:8:"page_url";s:51:"http://en.wikipedia.org/wiki/Golden-crowned_Sparrow";s:9:"permalink";s:84:"http://en.wikipedia.org/wiki/Special:ArticleFeedbackv5/Golden-crowned_Sparrow/155433";}
job_timestamp: 20120709104829

I currently can't dig deeper into this matter. Up to the point of calling UserMailer::send(), everything should be functioning well.

Roan: I'd suggest looking into what wfDebug() has logged and/or mailserver logs, as this seems to be the point it's failing.
If I can do any of that myself (or a brighter idea hits you), please let me know what & how ;)

Additionally, Fabrice: the current email to oversighters contains the copy to be found in requirements, plus at the bottom:

P.S.: If you have any questions about using the oversight features of the
article feedback page, please check this oversighter FAQ:

$4 (-> link to FAQ)

Please note that the article feedback tool v5 is still in early stages of
testing on the English Encyclopedia and that the feedback page has not been
publicized to the community for this testing period.

Should I remove this part, do we keep it, or would you like to modify it, now that AFT is gradually being displayed to more users.

I've tried simulating a mailer job on the command line on one of the job runners (mw1), and it seemed to have worked just fine.

$wgArticleFeedbackv5OversightEmails = 'roan@wikimedia.org';

$job = new ArticleFeedbackv5MailerJob( Title::newFromText('Test'), array( 'user_name' => 'Catrope', 'user_url'=>'https://www.mediawiki.org/wiki/User:Catrope', 'page_name'=>'Test', 'page_url'=>'https://www.mediawiki.org/wiki/Test', 'permalink' => 'https://www.mediawiki.org/wiki/Test?oldid=12345'));

$job->run()

And the e-mail arrived just fine. So I'm starting to think this might be on the queue / mailing list side.

Got it. The context indeed was the problem: feedback oversight-req'ed from an article's feedback page were being sent just fine, from central feedback page not so much. This was caused by the way the email (which includes a reference to the article) was constructed.

Fix has been pushed to Gerrit (https://gerrit.wikimedia.org/r/#/c/17068/) & prototype - to be monitored closely after deployment, but it appears to be fully functional for me now