Jack,

Exactly what you're getting at is not clear, because it's not possible to infer accurately what salient characteristic of rap you want the antecedent to have... unless you tell us.

I've guessed you're leaving non-voice parts out of the equation.
I'm guessing you're focused on the sound object, but I don't know what other aspects are important to your definition.

I suggest the possibility that if you're concluding the thing you're looking for is not there, then maybe you're looking for the wrong thing.

Rap as a verse form and as a cultural practice has extensive roots in America.

In my work on THE chanty GENRE (the capitalized words being very deliberate), I map the continuities in the verse forms between many different practices connected in some way with African-Americans. The versification is improvisatory or otherwise a display of personal wit or otherwise is manufactured to mimic those qualities. It gives status to the versifier as a "man of words" (Roger Abrahams' phrase for calypsonians). Identical forms are found among people singing to their group-labor at such 18th-19th century tasks as rowing boats, shucking corn, pumping a fire engine, hauling a halyard, etc. The same goes for dance and for games/play. It is the verse of hand-clapping games. It's the verse for military jodies. It's in the free poetry of Rudy Ray Moore. (I agree -- it's not there in Gil Scott Heron.) It's the verse of the minstrel songs that closely follow African American style and gets locked in as a popular style, to recur in the descendants like blues, rock and roll, and country. And so with rap, in the beginning, when rap was conservative in style. Hip hop was new in the sense these verses were recited over funk music, looped. The spectacular commercial success thrust this versification of a quite old fashioned, nursery-rhyme-y sort, back into the spotlight. The Rock critics pissed on it because the artsy values they had developed by that time made the old timey black American versification sound simplistic and regressive. That didn't stop rap from resonating with a huge percentage of Americans who have this in their national DNA. (Same goes for "Old Town Road" this past year.)

It's all there. I don't know what "folk collecting" has to do with it. "Folk" ideology rejects or has a huge blind spot when it comes to popular music, which is what most of this stuff is. So yes, it hasn't entered into the historical record so much through that lens. (I must object however to your implied linkage of "folk collecting" and ethnomusicologists. As an ethnomusicologist myself, my field's mandate to study "music as culture" -- a musical anthropology -- was set since the 1950s when the term ethnomusicology was adopted, and it's not about "collecting." )

I consider the chanty genre to be an antecedent of rap. But see, one has to recognize the actual verse forms, the use of language, the social gestures of performers, the cultural lineages of the chanters. People graft their impression of SEA SONGS onto "chanty" and the picture gets cluttered with a bunch of irrelevant sea-related nonsense. Similarly, when rap becomes "speech like music" (or whatever, you get people piling on whatever speech-y thing is in their frame of reference. Same goes for "jazz," ugh! How many times have I seen someone mention that a music tradition involves players improvising and an audience member goes "Ah yes, like jazz!" (No, dude, like the majority of musics that involve improv and have nothing to do with jazz!)

sorry for the rant. Easy in the morning. Take care! ~G