ON THE MARCH: White House aides are amused--and irked--by California Secretary of State March Fong Eu's drive to become U.S. ambassador to an unspecified Asian country. Ten thick packets have landed at 1600 Pennsylvania Ave., containing photos, brochures and letters of support from elected officials, academics and Asian-Americans. . . . Running the effort is Eu's deputy, Tony Miller, who would become acting secretary of state if Eu moves on. Working against her: Gov. Pete Wilson could name a Republican to replace Eu, a Democrat. Working for her: Asian-Americans claim they're underrepresented in the Clinton Administration.