The copyright for Benet's book is 1927 and 1928. Shay's book that contains the "maverick verse" was published in 1948. But it is based on an earlier version from 1924. However, this earlier work by Shay has no mention of the "maverick verse" and, it doesn't have the last verse which contains the line about the "Congo River". See here:

http://www.archive.org/stream/ironmenwoodenshi00shayrich#page/10/mode/2up

Berger's book, mentioned above, was published in 1941. So where did this "maverick verse" come from? I suppose it's possible that Shay got it from Berger, but that doesn't seem likely to me. Is Benet's literary adaptation/creation the source? But Benet's verse is supposedly from "Blow the Man Down" and not "Blow Bullies Blow". Perhaps Benet knew of the old New England whaling verse but switched it to another song for his literary purposes. Perhaps the verse was used in both songs. I seems unlikely to me that Shay and Berger would have taken this from Benet and changed the song. Benet is the literary person here. I would think that it is more likely that he adapted it. In any case, this verse, which could be an important link to the slave traders is clouded.

And with regard to the "Congo River" verses, here is what I have found so far. C. F. Smith mentions that they existed but doesn't quote them. They show up in the Frothingham collection mentioned above (1924). One verse shows up in the Shay revision of 1948. Doeflinger (1951) has "Congo River" in one verse and mentions that "this shanty recalls the old Guinea trade." (pages 25-26). Colcord says that this song "started life as a slaving song" and gives the Congo River verse ( p. 47).   And we have Hugill's version from his 1961 book, which he says he got from "an Australian seaman, ex-"Manurewa" and "Silver Pine". (page 226). Do we have any other references to the "Congo River" version?