I have not had the opportunity to confirm this experiment with anyone, and I do not know the precise details or possible ramifications of using this "mod", but for your information:
There is apparently a way for your computer to temporarily download any exterior model & textures that another person online is using no matter what the aircraft is. Default or addon. Both parties must have the following lines enabled in the [MULTIPLAYER] section of the FS9.cfg for this to work, but it could save us from suggesting that people stick to using MS default aircraft. The following lines in the cfg are already present, but with 0 values. Change them to the following:
ALLOW_PLANE_MODEL_SEND=1
ALLOW_PLANE_MODEL_RECEIVE=1
ALLOW_TEXTURE_SEND=1
ALLOW_TEXTURE_RECEIVE=1
Say for example that I am using Team KBT's P-3 Orion with a third party repaint for FS2004. That's a third party aircraft, with a not included third party repaint.
Suppose then that another person is flying online with me but does not have this aircraft or repaint. If we both have the FS9.cfg lines installed, then his/her computer may download the exterior model and livery straight from my system so he/she can see it properly.
This will obviously take up some bandwidth, but it might be worth it during small events.
I have my system set up with the FS9.cfg mod ready to go.
Again, I don't know how "appropriate" using this built-in feature is, mainly because I am not precisely sure how much of another aircraft this downloads (pretty sure that it's just a temporary session thing though, so you can't keep the bird), but I just thought that it is an item of interest.
-Sean