I'm pretty sure this will not qualify with 529. Unless the MCAT prep is held as a course in a community college or something.
From the IRS website:
Eligible educational institution. An eligible educational institution is any college, university, vocational school, or other postsecondary educational institution eligible to participate in a student aid program administered by the U.S. Department of Education. It includes virtually all accredited public, nonprofit, and proprietary (privately owned profit-making) postsecondary institutions.
Most of these prep courses are run by for-profit private companies like Kaplan or Princeton Review, and hence don't qualify as an eligible educational institution.
I don't think there're really any options but to pay it with normal (after tax) dollars