Hmm...

Here are the search results for "protest song" in 1899... none are of the two words together. click

Here are the search results for "protest song" 1950 to 1955.

Predating Greenway, there is "Folksongs on Records" edited by Ben Gray Lumpkin, issue 3 of 1950, in which someone calls "Strange Fruit" a "protest song against lynching" but says "I do not believe it is a folksong." (This appears to be referring to the 1949 album, "Josh White: Ballads & Blues, Vol. 1", Decca DL 5082.)

Other than that, it looks like it's 1953 with Greeway's publication and discussions related to it that it really kicks off in print.

~ Becky in Tucson