I did it once but when I wanted to reproduce that recently I could not remember how...
Tonight I gave it another try and I succeded!
I prepared a little "demo" with two ORIONs taking off from a sea level and a mountain level airport and I sent it to SUBGURU. Hopefully, Bill will upload it in the download section. In the .zip I sent Bill, there is a little txt. file explaining the few things you have to do once the mission starts...
I will prepare a little tutorial but in the meanwhile, feel free to download the "demo" from Bill's site and open the file with the mission editor. I still have some things to clarify but I'll run some test...