Pick a slot in the mission. Then go into the hanger and pick the ride you will use in the mission. Pick your skin and your loadout. Then spawn on the runway and wait for the mission to start. I believe you can do this and pick a completely different ride.
Bingo. However I have a barely slightly better method.
Go to mission field, pick a plane and loadout, wait for mission to launch in F5 view.
Wait for mission to launch so you can see which runway they're on.
When it asks how to score, launch your plane first, then select how to score it.