I appreciate the reply. It's a good thought but actually I've been playing on Death Incarnate for maximum carnage. I guess they must have replaced the ghosts with regular enemies (officers, SS and those floating Dracula Hitlers or whatever the hell they are). I can't remember if they were there along with the ghosts or not, but the ghosts themselves were creepy. As far as I know, they couldn't be killed and if you touched them, you took a lot of damage and usually would die.
Even if there was a copyright issue with the ghosts, it'd be nice if theykept them but replaced them with another skin. They really gave that secret floor a unique feel.