"when I die" phrase is meant to put death at arm's length, so to speak, so it can be treated jocularly".

Maybe so. But I still think that these lines have their origin in burial customs. I'm sure that I've read print and/or online accounts of [African people and perhaps-though I'm not sure] also enslaved Black people having bowels and cups and even some food buried with them. Here's an account of the documented practice of Black folks placing objects on top of the grave site [which may or may not be for the same reasons as burying objects with the deceased].

http://www.sciway.net/hist/chicora/gravematters-1.html

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Also, there are other floating verses from 19th Black dance songs that reflect the fact that violent, sudden death was a constant possibility for Black people during those times. One line that I've seen in a number of these song is "if I live to see next fall"

Here's one example:

SUSAN JANE
I know somebody's got my Lover;
Susan Jane! Susan Jane!
Oh, cain't you tell mel here me find'er?
Susan Jane! Susan Jane!

If I lives to see nex' Fall;
Susan Jane! Susan Jane!
I hain't gwineter sow no wheat at all.
Susan Jane! Susan Jane!

"Way down yon'er in de middle o' de branch;
Susan Jane! Susan Jane!
De ole cow pat an' de buzzards dance.
Susan Jane! Susan Jane!

Source: Thomas W. Talley "Negro Folk Rhymes" {Kennikat Press Edition, 1968; p. 77; originally published in 1922}