No. So the original problem was that I could have the dialogue with Madame Serenity, but she wouldn't open the door. I reloaded the save and went about my business. I came back a long time later and she wouldn't even have the dialogue with me (and I couldn't get it to trigger automatically in the back room either).

Now, I found a way to glitch through the door if she won't open it. Get on Roach outside the Passiflora. Ride him up the stairs and enter into the building, then ride him downstairs. Now, go to the door that leads to the left room. Ride him as far through the door until it stops you, then dismount (Roach should be about half way through the door). Run around the outside of the building and into the room which Roach's head is now in and mount him. If you get it right, it should allow him to pass into the room. Essentially, use the trick in the following video to work your way into the back room where the strippers are changing and where the secret door is.

https://www.youtube.com/watch?v=V5ufSikKwak

Once you have got roach into the back room, ride him into the corner where the door is. Using a series of random jumping and galloping, get Roach to glitch as far through the wall as you can. Then, turn him around so his back legs are stuck in the wall, not his head. And dismount. If you do this right, it will cause you to dismount on the other side of the secret door. The trick here is that Geralt will always dismount backwards, so you just have to get it perfect by luck.

Now, one problem. The way the quest is poorly scripted, the Serenity dialogue MUST be triggered before the next stages are unlocked. So, if you glitch through the wall before that, she won't talk to you, theres no point. Dijkstra and Roche will be up there, but they won't allow you to proceed with the quest.

However, before the special fight towards the end of the game, you have an opportunity to recruit allies. Roche and Dijkstra are two of them. I had to use the Roach wall hack to reach them in order to proceed, as the secret door was still closed.

TLDR; If Madame Serenity dialogue is broken, "A Deadly Plot is broken. However, you can still use Roach to glitch past the secret door.