What about giving each player his unique livery? This way, there would be no problem during the subscribe phase. However, with the current plane images, that would require a *huge* amount of graphical work...(hundreds of planes )

Unless...
Solution 1) Each player can upload his own livery for a certain plane. If he has no livery, he is assigned the standard one of the scenario.
That way, the graphical work would be transferred to players...do you want your livery? Make it!
(or ask some good fellow pilot to make it for you)
Solution 2) Planes are no longer a single image but they are split into engine, wings, fuselage, tail.. This way, we could just create 8 colours for each part to get 4096 different planes..
(It would still be a lot of work, but easier).