Due to the limited historical record we have available, it may not be possible to determine if Sam ever used the term "Nigger Jim" when referring to Jim outside of HF.
I'd venture to say, however, that Sam's actual pronouncement (or not) of the term has not been as influential upon modern usage as Hemingway's oft-quoted comment on HF in _Green Hills of Africa_, which refers to Jim as "Nigger Jim."
Jeff Miller
Gonzaga U.