Note: I do not believe that allowing the wearing of colander hats by military cadets would be deemed a reasonable accommodation by federal courts, even if there was no question of validity of the religion. As others have noted the hijab may or may not be a reasonable accommodation, depending on the specifics of its practicality. The reasonable accommodation language makes it pointless to come up with absurd examples, because if they are absurd enough to significantly interfere with the purpose of the activity, they don’t have to be accommodated.
Just to make this clear: a government entity cannot avoid the duty to make reasonable accommodations by simply saying that it never makes exceptions; it must rather show why making this particular exception would not be reasonable.