Ian_S

I think we may have all been barking up the wrong tree. The following photo, which I've only just found (online at the Tasmanian Archives, I thought I'd exhausted that source years ago but here we are ... ) shows the rope in question. It is hooked to the single-sheave block of the foremast boom tackle of the gaff sail rig. So it's a part of the running rigging. I think what is in all the photos I've shown would be called a boom tackle pendant (or pennant). 

It relates to an explanation of boom tackles (also called boom guys) in Chapelle's "American Fishing Schooners" (p.357): "Their purpose was to hold the booms outboard when running before the wind the vessel rolling." (sic, seems to be missing a word or two). In the photo I've attached below, we see the single-sheave block with a hook that if not in use would be hooked to an eye under the boom a little aft of the boom jaws. The other block in the tackle is a double sheave block, hooked and moused to a wye band near the sheet wye toward the end of the boom (can't see that in the photo, but Chapelle's diagram shows it, as do Figs 10-12 to 10-14 of Bennet's Schooner Sunset. In Chapelle's description, "To guy the mainboom, the single block was unhooked and brought to the rail near the aftermost fore shroud and hooked into a staple on the rail cap. The fall was then led off and set up, then belayed on the pin in the boom jaws.". So there should be a pin in the boom jaws in the photo below, but it's obscured, so you can't actually see it. 
 
Chapelle goes on, and his next comment reflects what is also clearly depicted in Bennet's Fig 10-13, which shows the British version of the boom tackle pendant (or pennant): "The foreboom guy led to a staple on the bow chock rail ... the fall belaying on a cleat on the side of the foreboom. Racers sometimes had pendants for guys to hook into, to avoid overhauling the tackles, which were very long on a large schooner." 

So, unless someone raises objections, there we have it. These southeast Australian skippers in their sweet picturesque old-time sailing vessels, the seas already full of steamers and motor vessels, when out on the seas far from land where nobody was watching, turned into insatiable speed freaks who held the booms as far outboard as they could, by extending the boom tackle on long outboard pendants. 
 
The photo below, in full and at higher resolution is at https://libraries.tas.gov.au/Digital/LMSS761-1-595

So we have it? I'd be interested if anyone can provide more detailed references to boom tackles.