Sounds like you need to do the unthinkable and go have a talk with a suspension expert and tell them exactly what is happening and exactly what kind of testing you've done and exactly what the conditions of the leak are.
Edit - Here's a little info why I think you could go talk to someone
It might not be the air seal. It might be the valve.
All Schrader valve lose air over time. All of them.
It might be wear on the inside sealing surface the air side tube.
It might be damage to the sealing surface on the air piston from someone getting too zesty with a pick while changing the seal.
It might be the wrong seal used in the air side.
It might be the right seal installed upside down.
It might be the seal that holds the Schrader valve in to the top cap.
It might be the seal around the top cap that seals the interface between the top cap and stanchion.