Is the reason this cannot be an automatic part of the spawn process that you cannot be sure everyone has that skin downloaded? If accepting a slot in the mission changes your plane back to the default each time when the mission launches. Then I would venture not having the targeted skin downloaded is close to the issue. But, still programmatically, the lack of the target skin not being available for the player would be an automatic substitution of the default skin.
This is too simple. Must be more to it from Hitech's side.