Michael Crowley has more, including some actual corroboration. Though it sounds like the feminism angle is growing more plausible. The kicker from a Washington Post story in 1994:
But Ann Henry, a University of Arkansas business professor and Fayetteville friend of the First Lady, said she “vaguely” recalled the episode in the context of frequent discussions they used to have about women’s access to supposedly open career paths. Frequently, the equal opportunity that was promised was not a reality — and sometimes female faculty members went out to conduct “tests,” Henry said.
If Hillary Rodham had asked about the Marines as a way of testing whether the corps actually welcomed women, “it would have been consistent with what was going on with us at the time,” Henry said.
“Is it possible she was testing?” Blair asked. “I don’t remember if she was seriously exploring a career, or was moved by curiosity, or patriotism or feminism. I wish I had kept notes.”