Well the long flights (read: transatlantic, transpacific, etc.) using dual-aisle aircraft always have changing tables. For some reasons, the single-aisle vs double-aisle seems to be the dividing line (more or less) with many of the American carriers with regards to whether or not there are changing tables.

As to what I do: I'll be honest and say I figure it this way... My kids were always good sleepers (knock on wood), and they wear the special overnight diapers for 12 hours a night while sleeping without being changed midway through the night.

I am generally a religious diaper changer, but for one day out of a million when traveling, if we're talking about only pee diapers, letting him and/or her wear one of the overnight diapers (which I apply immediately before boarding, so it's plenty fresh, and change immediately upon arrival) for a 6-hour BOS-LAX flight is less time-in-diaper than he/she does on a regular night. I know the diaper is up to the task, and I know that baby won't feel the wetness because those overnight diapers absorb like heck. So basically, I let them be so long as they don't poop. I pray they don't poop.

If one of them poops, obviously the diaper is changed immediately either in a manner like you had to do or once they can stand, I do it with them standing.

No matter what you do, it's not easy and not super convenient to have no changing table available, but with luck you can get through it the way I've described or hopefully with no more than 1 onboard change. Once they can stand, it's SO MUCH easier.