Happy lot here on flyer Talk, aren't we?
I might choose the end of the flight rather than in the middle. But, being as airplanes are a strong interest of hers, then I think it is rather romantic.
I do have to agree, though, that I would make absolutely sure she is going to say yes.
Would there be a way you could find a rather short flight, where several family and friends might be able to fly along? You might need to contact the airline about getting on board somehow without her knowing the rest of the family is there.