Marlow
Marlow Ropes can do all sorts of fancy custom stuff with factory-tapered cores, changing weave patterns, and adding different materials in as the line progresses. The guys there are very friendly and helpful, and even if your budget won't allow for custom-woven halyards (I'm sure they're $$$$$), they might have some good suggestions.
But surely the spar maker is in position to give sufficient clearance? It seems that if one is designing a 5/16" hole, a 3/4" hole would not be hard to achieve?
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