The 2-3 step process where the user is actually clicking a submit button is used by most advertisers. Go look at diet and bizopp offers and you'll notice they only collect basic info up front, but once again the user is clicking submit.
If what I'm talking about isn't possible to code, which I find hard to believe, then the only other option would be to record the users screen with something like mouseflow or clicktale, but then you need someone to watch videos.
There has to be a way to capture the data as it is typed, and I'm sure some of the major advertisers have users all the time that fill out some basic info, never hit submit and leave.
Not sure how the user would feel though if you called them or emailed and they knew they never hit submit.
Can be done with AJAX pretty easily, it's a similar principle as instant form validation while the user types
IMO the issue here is the person technically didn't give you consent to use his data since he abandoned the form.