I have to confess I'm scanned a lot of early 19th century journals of old sailors looking for references of sailor's work songs. Can't find anything earlier than Nordhoff for describing the song leader as a "chantey-man." In his case I believe he was describing the work task as loading bales of cotton in some southern port, rather than work aboard ship such as dealing with the anchor, the sails, or pumping.

It could be that the term was adopted from the loading crews from dockside and extended to describe the other already existing sailing work songs.

Cheerily,
Charley Noble