There was an NFL player who quit his job and joined up to fight in Afghanistan. But realistically, it would be asking a lot to expect some pampered athlete to walk away from millions of dollars to go fight in the military and put himself in harm's way.
If you think about it, their salaries *are* helping to fund the war because they are rich enough to get taxed at the highest percentage.
As for other displays of public support, I don't doubt that, just as with charity, their good works often go unreported and they'd probably prefer it that way.