Schools have to deal with these types of issues in many environments, and so they're used to negotiating solutions acceptable to families and other schools. The idea is to get to "yes" for the student, not to enforce a code. The code is a tool for them.

The NCAA is used to declaring the Commandments of Collegiate Sports from on high, and having eager colleges and universities, who make a lot of money off of college sports, fall over themselves to comply. For the NCAA, the code and its administration is about half of the reason for their existence.

Also, I haven't noticed a lot of flexibility among conservative religious people of any denomination when it comes to things like this, unless they are offered some concession to begin with. Their sense of defending their public expression of their religious beliefs precludes them beginning the process of compromise, though once the process is initiated by the other side, I notice them joining enthusiastically. The show of respect for their beliefs is very important; after seeing that, reasonable negotiation is usually not difficult. They see themselves, and correctly I believe, as an embattled minority who must not yield first so as not to appear weak.

I suspect that a headcovering made of some kind of breathable spandex, that acts and looks like a swim cap, would stay put during a game, would not act as a distraction to players, and would not impede the young lady's ability to play. People wear sweatbands all the time; this would be much the same thing, only for the whole head.

In league soccer, girls are not permitted to wear any accessories made of a hard substance like metal or hard plastic, because of the danger of injury from impact with the ball. This includes hairpins, barrettes, and earrings, even studs. (An earring post can give a girl a nasty little puncture wound in the neck if the ball slugs her in the ear.) So any headcovering would probably have to be secured with very good elastic, and not hairpins, but we have very good elastic these days.