Great idea, which I would personally love to see. Especially changing plane numbers.
However...
With regard to "doing it like noseart", there are complications:
Squad noseart is limited to a 64 x 64 pixel bitmap in 256 color. Not huge or resource hogging in the normal scheme of things but still something that has to be rendered by your computer every time you come into contact with a new (since you logged on) squad. The other "issue" is that the background transparency has to be {176,176,176}. This is so your noseart "blends" into the background. By this it means that, if your squad art is not an exact 64 x 64 pixel square (say it's a circle, triangle, dodecahedron, ellipse, etc) whatever part of that 64 x 64 square that is not colored will be "invisible" on the plane, otherwise whatever is left of that square would still show up on the skin. Essentially, you would have a white square on the plane with your squad art in the center and it would look silly.
This may work for other, smaller things (such as a triangle like clerick suggested), but would not work for the numbers on the planes. For instance, Luft numbers are around 100 to 150 pixels (square). If you made the background transparent(like the noseart) your number would be superimposed on top of the existing number on the plane and the number that exists would still be visible behind the one that you picked. You can see this example with squad art on some planes. If you look, the player-submitted noseart does not always cover the noseart that is on the skin itself and the default one pokes out behind the "AH squad" noseart. I can't tell you which ones this happens to off the top of my head, but if you look at it on enough planes, you can see what I mean.
The only way that choosing numbers like this would work is if EVERY skin, default or player-submitted, did not have an existing number on it but that would not comply with HTC's policy of submitted skins being historically accurate.
It would be a great idea for squads so that each member could plaster his flight designation on the plane but, unfortunately it wouldn't work, at least not in the way that the noseart currently works.
The only other viable alternative that we now have is to research planes for the historical squad that you fly for (if it is a historical squad), find pictures/references and either skin the plane yourself or ask a skinner to do it for you.
