Amos, nice summary of the little available.
The word is not in Lighter, "Historical Dictionary of American Slang," or other references I was able to check.

According to this history of Arhoolie Records and its founders, Mack McCormick of Houston, TX, manager of Lightnin' Hopkins, suggested 'Arhoolie' because it has "a ring somewhat reminiscent of a field holler." Arhoolie story, part 2: http://www.bluesartstudio.com/NeueSeiten/pageA58.html
Arhoolie