I nursed my then-15 month old
while he was sitting in a carseat on the plane, on multiple flights, this summer. No one ever said anything. (BTW, purchasing an extra seat for him and bringing our car seat was the BEST decision we ever made! It made things so easy and comfortable.)
We had the last row 2 times (which is nice) but for 2 flights we were in the middle of the airplane. No big deal! The people behind us loved playing peekaboo! Not while I was nursing mind you (all they could see was my back, I'm sure) but when he woke up, they loved him!
PS. I can nurse him in a carseat and stay in my own seatbelt. If you couldn't do that, then I wouldn't recommend it, as it wouldn't be safe. I guess I have magical (read: floppy and big) boobs