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.

Look, honestly....I like your creativity. But I would NOT use that

I like the 'will you marry me' part but I've never been a fan of opinion openers.. if your knew I guess they're a good way to start without too much approach anxiety but just back yourself man.. Go in there ask if they'll marry you and then laugh and go they're not ur type and u just won a fiver from your mate for saying that and just get chatting to the girls..
Opinion openers are just a bit forced imo.. But yeh I reckon I'm gonna use that first bit thanks x

So your using getting marry as an opener and flossing thing as routine. It can work.

If your gonna be edgy with your opener, sometimes your gonna get IOI from girls right from the start. In this case, you need to drop the negs. Stick with teasing instead. Go out and practice and learn how to calibrate.

Two question/openers one after another feels more like you are making them take a poll. Stick with one question, and expand the subject. Instead ask something like "why do you think people marry so young?"

I've seen a video a while ago of two guys using this opener. They are doing a great job as a team, but I think if you're going to use this opener you gotta be 150% confident, else it'll fail.

Video :
[youtube]http://www.youtube.com/watch?v=FK6uA3xiiW4[/youtube]

_________________
I am me

Thanks for the feedback and advice, guys.
@YDK: How do you go about initiating a conversation after claiming to have just won a bet?

@Datguy: Trouble is, with a question like that, I'd feel like I had to justify asking it, e.g. "because my friend is only 19 and he's engaged". I don't like to lie about large events, a conversation is fine but creating an entire person with a problem feels very awkward to me.