I can't imagine he's talking about a steel core for his saw lanyard. I'm guessing he wants some steel holding his body to the tree in case his saw gets a little out of control?
Dave - If I'm right about what you're looking for, steel core lanyards are available from any tree equipment dealer. No reason not to use for your second tie in unless you're near electricity. And you shouldn't be anywhere near electricity. I've heard people talk about putting a wire on the end of their climb line, but I can't imagine any way that would be practical.
It should be noted you can still cut through a wire core, although it does take a bit more work and you'll probably feel it in the saw.
Be safe.