Masquerader achievement in AC Mirage

You can get the White Patient Robe just before you kill Al-Rabisu at the end of main quest The Great Symposium. I'm not sure if you're able to kill him without it, so I'm not sure if this disguise is missable. However, because the other disguise, the Eunuch Tunic, which you get while investigating Qubiha's Domain during the main quest, The Servant and The Impostor, is required to gain entry into the Harem, the Eunuch Tunic is completely unmissable and story based.

Edit: Okay, so according to JJR0906, this achievement is missable because it's entirely possible to kill Al-Rabisu without finding the White Patient Robe. If you go back, it's not there.