How about a state law against branding or tattooing infants or any other irreversible body modification?
Clearly these are both rhetorical questions with obvious answers. Whether or not those obvious answers are morally correct is another matter.
Ergo, while traditional, the fact that you are requesting the modification of another human (not yourself), I could see this not surviving a challenge down the line.
After all, female genital mutilation (which is obviously far worse) represents a similar request for modification of a living human - not yourself - for religious purposes, and it is outright banned in the overwhelming majority of the United States. While a Federal ban was struck down, it was not struck down on First Amendment grounds but rather due to same being outside the scope of enumerated powers of the federal government and not covered by the commerce clause. [1]
This leaves the state laws in force in all but 9 states.
[1] https://en.wikipedia.org/wiki/Female_genital_mutilation_in_t...
Some people think it would not survive a Due Process challenge, though, because there isn't a strong enough basis to overcome a parent's fundamental right to make decisions about their child. I could see that being a close question.
The case I could make is that it may prevent the child's right to future free exercise of a religion in which the, uh, package should be, err, OEM.
Once you turn 18, by all means, feel free to take a little off the top.