All right, now that the dust has settled a bit and the community has had some time to investigate thoroughly, the closest thing that we have to 100% confirmation right now is that all of the "unknown" navigators from my original question [can be earned from Score Attack][1]. (There are scattered reports of other methods, such as [mike's answer][2], but none that have been verifiably proven to work consistently that I'm aware of.)

Therefore, the closest thing I know of to an authoritative list at the moment is as such:

>!Teddie: Clear Story Mode completely<br>
>!Mitsuru: Complete 100% of one character's challenges<br>
>!Nanako: Beat Score Attack mode with Yu, Elizabeth, Naoto, and Shadow Labrys<br>
>!Aigis: Beat Score Attack mode with Aigis, Labrys, and Mitsuru<br>
>!Fuuka: Beat Score Attack mode with Yukiko, Akihiko, and Kanji<br>
>!Yosuke: Beat Score Attack mode with Yosuke, Chie, and Teddie

For those who are absolutely dead-set to earn these unlocks by hand without resorting to spending money (or who wish to earn the achievements associated with them), note that you can slowly [lame out a win against the AI via liberal application of the pause button][3]. (Simply pause the game to see what move the opponent is performing, react appropriately, then repeat.)

I'll edit this answer if Atlus ever releases official unlock details.

  [1]: http://mayonakamidnight.com/showthread.php?1715-Navigators-Announcers-requirements&p=57014&viewfull=1#post57014
  [2]: http://gaming.stackexchange.com/a/80043/26052
  [3]: http://www.youtube.com/watch?v=NkhwJ-nXYPk