Women always have a boyfriend, the minute they finish with one, they are already going out with another, this would imply that they have to have a boyfriend, but exchange them whenever one they prefer the look of comes along, which in this instance might be you.
If not the above, they certainly must have a waiting list of suitors, so best to ensure you are on it in advance.
Maybe you could say to her, "Look, I know you already have a boyfriend, I just cannot help feeling their is something special about you, if your circumstances ever change, maybe you would consider talking to me further?"
If she does like you, she will either put you on her list, or maybe even decide to move up a model.