I understand that there were a lot of people on the ship during the "open House" but how did a Bajoran, who was not part of the crew, go unnoticed? Even Seven, when she first met the other drones, should have noticed that. Or how about the Security people? Surely there were someone checking people in?
I imagine that her nose ridges weren't that noticeable. Perhaps there are Delta Quadrant species with similar ridges? Perhaps the ship's sensitive areas were secured, so that the rest of the ship was available to anyone, and that's why she got on board unnoticed? Maybe the crewperson manning the airlock(?) was just not familiar with Bajorans?
Considering how many aliens there are out there identical or nearly identical to humans, I would not be surprised if there were a few who look a lot like Bajorans. Or maybe she was noticed and people didn't know what to say:
Voyager was infamous for not letting continuity get in the way of a story. They wanted a Bajoran for some reason, if only to save on makeup design, therefore they had one. The story is about how Seven is the Borg Queen. How can you reasonably cavil at anything else, no matter how self-contradictory?
I noticed that one of the drones was a Bajoran right away. We already know that the Borg had assimilated a vast number of species (millions of individuals from thousands of species). Couldn't it have been possible that they assimilated a Bajoran in the Alpha or Beta quadrants and used their transwarp technology to bring them back to the Delta Quadrant? OR the woman is part of another species that resembles a Bajoran. OR she could have been aboard the U.S.S Equinox...took a shuttle somewhere...got assimilated.