The other reference I want to share is to the same song used in a corn shucking context.

Hentz, Caroline Lee. _Linda; or, The Young Pilot of the Belle Creole._ New York: F.M. Lupton, 1881.

This novel was written in 1848 (published 1850). The author, born ca.1801/2, lived in North Carolina, Tennessee, and Alabama at various times in the 1820s-40s.

An extended quotation here. Scene from a Louisiana plantation. Also features some form of "Sittin' on a Rail." Pp. 157-8.

Soon she saw a torch-light glimmering through the trees, and she found herself near a large corn-crib, from which the choral strains were issuing. To one unaccustomed to such a spectacle, nothing could have been more picturesque lhan the scene that presented itself to Linda's eye. Large, pine torches were flaring near the door, and threw their red light on the black visages of about forty or fifty negroes, sitting in a ring round an immense pile of corn, on which was seated the sable master of the ceremonies, who was tossing the corn down to the group below, who seized it, one by one, with a yell of delight, and, squaring their elbows and shrugging their shoulders, they vied with each other in stripping off the dry husks from the golden ears. The African monarch of this harvest festival, as he threw the grain into the dexterous hands of the workmen, rolled out a volume of voice that shook the pine-boards of the crib, and every negro joined in the chorus with a vehemence and glee, a physical joy and strength, which none of the pale race can imitate—

"As I went out by the light of the moon,
Merrily ringing this old tune,
I come across a big raccoon
A sotting [sic] on a rail,"

shouted the Agrarian king; and then the sable orchestra chimed bravely in—

"A sotting [sic] on a rail, a sotting on a rail—
I come across a big raccoon
   A sotting on a rail."

Then, as the spirit of melody waxed stronger, the master would vary his strains, and—

"As l went down to Shinbone alley,
   Long time ago,
To buy a bonnet for my Sally,
   Long time ago,"

echoed through the woods, in one full, deafening chorus, dying away only to be repeated with more Herculean vigour. There is nothing that bears the name of music, that can be compared to the negro's singing; he sings all over; every muscle quivers with melody; it gushes from every pore The sounds seem to roll from the white of his eyes, as well as through his ivory teeth. His shoulders, elbows, knees, all appear instinct with song. He winks, he grins, stamps with his feet, taps with his heel, pats with his toes, raps with his knuckles—in short, gesticulates in every possible manner the human form admits. Oh! he is in his glory at a corn shucking!

I get the feeling from the tone of this that the author would not simply have been quoting T.D. Rice's popularized version--which suggests this was the vernacular tradition on which Rice based his song.

These references don't explain "bully in the alley," but they point to the existence of a similar song in an Afro-American work context before sailor chanties became widespread.