My Dh is a pilot for US Airways, I am a former flight attendant for them, well the America West side. I had my DH look in a flight attendants manual not too long ago, when there was some issues with carseats on the plane and FAs buckling down on what the airlines were telling them were the rules. Per their manual, the seats are to be installed according to the manufacturers instructions, which means if the seat is FAA approved and can be used to 65 lbs. in a car, the it can be used to 65 lbs. on a plane. I wish I could get my DH to make a copy of the FAs manual, but he doesn't have a copy machine on the plane, so that would be really hard. I have emailed my uncle's ex-girlfriend, who is a current US Airways FA about this, getting manual page copies, and haven't heard back from her.
If they pitch a fit, and you feel like arguing, ask them to get out their manual and show you where it says that the seat cannot be used after 40 lbs., on the plane.