I've never had to show proof of airline/flight and have used different codes many times. Iberia return is £27.20 at the moment. ANA is £27.52.

My flight tonight is an Iberia codeshare, so justified, but they never ask.

Even in the days when you had to enter a booking reference to get the Virgin 10% off and free upgrade to First Class, I put in BA ref numbers and it was fine (I believe the system just checked to make sure the same ref wasn't used twice -any 6 characters would do).