There must be a theory here, I’m just curious.
Having upper and lower lifelines, or upper, intermediate, and lower lifelines on boats bigger than a BCC, is all about the vertical gap through which an object/body could pass with ease.
Check the World Sailing/Offshore Special Regulations for their specifications for working decks.
For most categories of monohulls, the upper lifeline/pulpit rail has to be at a minimum height above the deck of 600 mm (24 inches). Higher rails/lifelines are allowed.
The minimum vertical gap (so as not to let a body/significant object pass with ease) is defined as 380 mm (about 15 inches).
The Offshore Special Regs have a loophole for hulls laid before 1993. For pre-1993 hulls, the vertical gap can be as large as 560 mm (22 inches).
I’ve seen nothing in the Offshore Special Regs that precludes a BCC from running a higher safety line (such as an old/retired halyard) from the upper corner of the boom gallows for’d to a shroud (my meaning being that such a safety line is NOT a fixed lifeline and so the vertical gap between that safety line and the upper lifeline is not an issue).
We do it all the time. We love um!