That's what I love about this forum. Every once in a while we get new and "impossible" situation.
I don't believe it's possible so saw or rub through a BB shell with a gear wire, but here it is. I suspect that it was more of a rusting process with the wire serving to hold water against the shell, letting the rust etch it's way in.
BTW - a few good closeup photos would be nice.
Clean it up as well as possible, and close it with body filler, or a "metal" epoxy. But you'll need to reroute the wires, so I suggest screwing or gluing a steel or plastic guide.
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