I don't know if this is possible or not, and I doubt it'll happen, but here's an idea.
For three days from September 11, 2001 to September 14 in-game, all flights to, from, or within the United States stop. No money is made off of the routes. Nothing happens. Then on September 15 in-game, things pick back up.
...or is it a "too soon" sort of deal to do that?