Hi L.,
I'm a first time mom to a 7 month old girl. We've flown twice since her birth and plan on doing it again this 4th. Our daughter has also only had breast milk since birth.
Both times I ended up nursing on the plane, although I had planned on bottle feeding. As for the other passengers - I really don't think they noticed. (Frankly, I don’t care if they did. I decided while in the hospital - about the time the nurse was manhandling my breast to show me how to nurse – that with motherhood all modesty had left the building.) I sat with my daughter in the window seat and my husband sat next to me. I also wore a cover while doing it. It's a new experience for them and it might be more comforting to him to be able to nurse as oppose to bottle feed - trust me you will do just about anything to keep them from howling on the plane. If you succeed, the other passengers won't care if you had to whip out the girls to do it.
The second time I flew alone with her; I told the stewardess about my situation and she found another seat for the person who was seated next to me. He was more than happy to be reseated as even a 2 hour flight next to an infant is not usually on the top of most people's travel picks.
As for keeping the milk fresh (you'll want at least a days extra with you in case your body decides the trip is too stressful and refuses to make milk). We packed frozen packets tightly together and had ice packs on either side. I carried it on and explained to the TSA people that it was breast milk. Since it was with the pump they didn't give me any grief about it.
If you decide to bottle feed on the plane and the duration does not allow you to just bring fresh, bring the frozen and thaw some out in the terminal bathroom before. Or, bring your pump as a carry on and pump right before the flight takes off - that will give you at least 3 to 4 hours.
Another tip, I have a giant duel action pump that I've brought both times. The second time I managed to leave it in the rental car and had to wait over a week to get it back. For our next trip, I'm just bringing a compact single pump. It is still pretty powerful and it's half the size. Also, if it gets left behind, not as big a deal bc it was only about 35 dollars (Target I think).
Good luck on your trip!
J.