This is how the quest is supposed to work:
However, since slaves immediately vanish upon being freed, it's impossible to talk to them afterwards. Last count, I've freed 97 slaves, and I still can't get the "Twin Lamps" code phrase. I've been to every last dungeon in the game and there are no more slaves left to be freed, except those in cities and slave markets.
I asked already in the general forum, to no avail. I'd looked for any mention of this quest in other threads, and found that some have brought it up, but they all seem to imply that they've managed to get the code phrase already. Speaking with Ilmeni Dren doesn't have any effect, as asking her about "business" is a dead end if you're not a member of House Hlaalu. I've already dealt with her in the past, when I returned her glove for the Imperial Legion quest.
Strangely, as a member of House Redoran, I was able to speak to Galyn Arvel at Ald Velothi while on the stronghold quest to find wives for the settlers, and she said she would send over freed slaves, even though I did not have the "Twin Lamps" code phrase.
How exactly do you start the Twin Lamps quest? I looked in TES4edit and, according to the flags, the quest is supposed to be available from the start of the game. Freeing 5 or 20 slaves didn't work. Talking to Jobasha didn't work, as the topic never comes up.