Okay... I looked for Ashbringer back in Vanilla WoW for more nights than I would like to admit. I got really deep into the Warcraft lore because of that search, I looked into every hidden place and unfinished place, I tested every theory out there including that one you linked to: I got quite a few ideas about what we are looking for. But I leveled a Monk first, so I might not be eligible to obtain it myself as my paladin is only level 100 still.
The Ashbringer is 120% connected to the Scarlet Crusade, and the forming of the Scarlet Crusade. You cannot say one without the other.
So the theory:
Have anyone tried equipping the Scarlet Crusade Tabard and Ashbringer, then going into Scarlet Monastery?
The Ashbringer is "Blade of the Scarlet Highlord" after all. As was the flavor text on the original-but-never-available-to-players version of the weapon. http://www.wowhead.com/item=13262
And the original Corrupted Ashbringer from Naxxramas 40 man did trigger quite the extensive roleplaying event inside of Scarlet Monastery, something that was unprecedented at the time. https://www.youtube.com/watch?v=m3E6W6-rRGA
They added a new NPC for Fairbanks in 7.0.1 http://www.wowhead.com/npc=91266 Fairbanks was heavily involved with the Corrupted Ashbringer event.
I know they added a book for priests there. So they did alter something in Scarlet Monastery with 7.0 for sure - could be they altered a lot more than a bookshelf.
Also I think and have always thought that the Slime mob IS Timolain, his remains at least. I think we might have to make his phylactery as part of the hidden quest. Which means we are likely looking for his spirit or ghost or a grave. I would love it if Timolain's ghost sent us looking for his long lost wagon of magical flasks, and then go to Hearthglen to create his phylactery.
That was the main theory.
Some other thoughts I had:
And maybe the NPC in Darkmoon Faire is related? She does say the Scarlet Crusade will rise again. Maybe only the cropped tabard works?
And finally, maybe traveling back in time is the answer. Caverns of Times: Escape from Durnholde does feature a lot of the characters linked with the Ashbringer, there even is a mini event showing the crystal of the Ashbringer being purged and where it came from.
I would try it myself as I do love to explore and look for hidden stuff in WoW, but I dont have the Tabard on my Paladin, and I'm not level 110 with it yet.
I hope in that wall of text there was a clue that leads to the discovery of the hidden skin. Happy hunting kindred spirits!