It's based on the "when should you floss" opener, but it could go with any opinion opener.

Okay, so you approach your target (I thought maybe a two-set for this) and say completely seriously:
"Hey, I've got a quick question for you, the answer could change your life forever - Will you marry me?"
Then hold it for a moment and let the shock manifest a little before you laugh and say:
"Just kidding, you're not really my type... But in all seriousness, should you floss before or after you brush your teeth? A friend asked me a while ago and it's been bugging me ever since."

My thoughts with this were that it will give the shock factor when you ask her to marry you; it's probably not the kind of question she was expecting and the question is outrageous enough that it seems obvious you're kidding afterwards (you're not just saying you're kidding because she seems like she'll say no) and thus displaying a sense of humour and a little daring in a relatively safe way.
After that, there's the neg (said a little playfully, so as not to give too much offence), followed immediately by the request for an opinion so that she doesn't have time to respond to the neg, but she'll dwell on it. My aim here is to get her thinking: "why am I not his type?" and maybe try to pick me up instead of the other way around.
I thought it'd work best on a two-set, where she's in a more comfortable situation for the joke question and will maybe share a shocked expression with her friend, thus giving you more time to let the tension hang but posing less risk that she'll answer the question. I also figure that you've got a higher chance of getting an opinion, and therefore a conversation, in a two-set.