There was Frederick Douglass. Drawing on his significant book, Peter Myers succinctly describes Douglass's greatness here, on the Heritage Foundation's website. Douglass's evolution from a despiser of the Constitution to a defender of it, even without an anti-slavery amendment, poses a model for Americans today who seek a return to the Constitution. Douglass is required reading for the Tea Party.
Also before King was Jackie Robinson, the Dodger star who integrated the All-American game. Like sprinter Jesse Owens in the "Hitler Olympics," he showed excellence in his talent and in that way made the case for equality. In their own way they made, like King, natural law arguments for equality.
UPDATE: The Sage of Mt. Airy reflects on his own white, Southern tergiversations regarding the appropriateness of the King Holiday. Unlike Douglass, King apparently declined in his esteem for the document.