For a list of guilds that are found to have downed bosses via publicly-accessible Achievement information, a WOW fansite is available:
Sentinels 10-man progress
Sentinels 25-man progress
(( I don't have any information on whether guilds show up on GuildOx if no one has "added" them. ))
My account subscription will run out on 11 March 2013. The interest in this thread has been minimal, so it's entirely on someone else's shoulders if they feel a new version is needed. For my part, I'm going to ask that the sticky be removed and just let this one drop naturally.