something i thought of as i started my game, what if modded plains could fly in the background with the ACEO default plains. would be really cool to see.
No, it is impossible to code and compile. It is a no. Period.
First the mods you will have to select which ones you want in the game after seeing the default planes and the planes would then have to somehow show up on the background. So it is a strait answer that it would not be possible in ACEO.