Thanks to Joe from right here at the Forums, we have a lot more answers.
On the Hunter 356, the compression post, chainplates and arch all have ground wires leading to the keel studs.
The connection to the arch is behind the headliner access panel on the starboard berth headliner (refer to owners manual page 44B).
I'll need to double check that the factory ground wire is in place, but my theory is that the previous owner's boat yard didn't know where the wire was, so they added one. But, did it sloppily.
Thanks Seadaddler for pointing me to Joe!