The tradition of spontaneous rhyming to a rhythmic musical background is actually rather common in a lot of black music forms. Some examples are:

1. Bahamian gospel chants. "I Bid You Goodnight" is a good example; Jody Stecher just released a good CD of singers he collected in the Bahamas in the early 60's, called "Kneelin' Down Inside the Gate; it's a bunch of cuts that didn't make it onto "The Real Bahamas" LPs he did for Nonesuch.

2. Calypso. Calypsonians spontaneously improvised long rhyming songs/recitations on topics of the day.

3. Yazoo released an LP/CD called "The Roots of Rap" with all sorts of neat talking blues, track-lining raps, gospel and work hollers, all of which are in the style of a singer lining out a (usually) improvised song to an accompaniment.

The nice thing about all these is that they lack the annoying, throbbing drum machine, and the violent, sexist lyrics common in today's commercial rap (although some of the blues stuff gets pretty raunchy...)