I have Tales of the Sword Coast version 1.3.5512.
You might perhaps test your own game to see if this bizarre quirk is present in your game as well: CLUA yourself to Candlekeep, check out the pantry, kill the Iron Thronies, check out the pantry again. There's gotta be something that opens that passageway, I can't believe BioWare would bury this particular script in the Baldur.exe or something like that.
Well, I teleported to AR2608, and I see the area on the wall that you're talking about. I went to the third level, killed the Iron Throne people, then got caught and sent to jail. When Tethtoril teleported me back to the secret library, I CLUA'd back to AR2608 but I still was not able to activate the passage. I will try one more time, but this time, after killing the Iron Throne members, I'll transport back down to AR2608.
Even if the trigger isn't present in some or all versions of the game, it's not to say that it's impossible to restore it. I just don't know how, s'all.
EDIT: Still couldn't recreate it, sorry.