I wonder also how the word "chant" might have been pronounced in various Afro-Caribbean (or "Afro New Orleans") dialects. If it customarily came out as "shant," that would be one mystery solved. Far-fetched perhaps, but worth looking into.

Abrahams (DEEP THE WATER) cites a useful reference. It comes from the WEST INDIA SKETCHBOOK by Trelawny. Abrahams (probably acccidentally) cites vol. 1, but I find the reference in vol. 2, which has a publication date of 1834. Trelawny is describing a plantation scene of slaves at harvest time. He gives a song, with music notation. It is not 100% clear, but it seems like the song is accompanying light work. That is, although he casts the scene as one of joy and celebration, I'm sure work was also going on...though that does that mean this was necessarily a work-song.

In any case, the point of the citation is to show the use of the phrase "Chaun fine." Abrahams believes this was the author's way of spelling "shant fine," which he appears to claim (pg. 14) was still a phrase in use by his informants.

Indeed, I'd have read "chaun" to suggest the same pronunciation (i.e. like the name "Sean"). Cf. also my wondering, above, about the term "chaunt" for African American genres, though it appears "chaunt" had much wider usage for "song" (though perhaps with certain connotations) in the early 19th century.

Here is the passage from Trelawny:

Chaun fine

Elsewhere in the book, I see that Trelawny uses the phonetic spelling of "chaun" for dialect pronunciation of "shan't". "...aw chaun wary no mo" (pg. 16), i.e. "I shan't worry no more." It seems pretty clear to me what he was trying to convey, then, in his "chaun fine" -- a word that sounds like "shan't," but which obviously had a different meaning (i.e. it is a verb, in the imperative form).

In volume 1 of the book, "chaun" is also used to render "shan't" (pg. 307).