I have flown a couple times, wearing diapers for the flight. In this case, I ALWAYS wear plastic pants over my disposable diaper... I have had disposables leak at the leg bands, and the plastic pants kept my pants AND the seat dry.
Also, if you pack diapers in your checked baggage, I'd also put a couple diapers in your carry-on baggage. I was on a flight once that was delayed several hours due to weather. They ended up taking us off the plane, and told us that we'd have to catch a flight the next morning. However, we could NOT get our checked bags back, they sent those ahead on another flight. (I know, crazy..) Fortunately, I did have a couple diapers in my carry-on case, or I would have been up a creek until the next day!