Very interesting!
Do we have a firm date on the hymn, "Say Brothers, Will You Meet Us?"? Posted in thread 32038: Civil War Songs
One website says 1856-1858 (the Steffe story), and another says it appeared in 1858 as the hymn (copyright and/or sheet music), or "Oh, My Brothers Will You Meet Me?" (Union Harp [?] but I have no means of checking. See Say brother
Some of the information is questionable. The sheet music I have is from an article published much later.

According to Lois Eagan, a letter from Port Royal said the song was sung by slaves "at General Drayton's plantation," in a "shouting exercise" to the tune of "Say, Brothers." "Sisters, soldiers, preachers," etc. were given an invitation to meet on "Canaan's happy shore." Anecdotes
I cannot find a date or further details about this 'letter'. According to a note (p. 259) in Dena J. Epstein, "Sinful Tunes and Spirituals," the song may have been heard at St Helena in 1862 by Laura Towne.