Oh, I've heard it - from my mom (I'm 38). She lived in Mississippi in the mid 50's so maybe that's where she got it. I don't have any idea if it's the "original" version of the rhyme and the two who filed the lawsuit are just pulling the latest version of the Tawana Brawley scam.
I've never heard that the rhyme comes from slave days. I doubt these Plaintiffs did either till they talked to a lawyer.