Yes, FacelessMike, the chorus was sung by a member of the band. In lieu of his real name (I have no idea who he is), let's call him "The guitarist on our left/Scott's right."
It wasn't Mark, of course, but it was more than adequate. I forgot to mention this also--Scott doesn't hold the "Ohhhhh" note underneath "So I held my head up high..." the entire way--he stops midway and takes a breath. It's probably not important but I did notice it, and it still sounded fantastic--even more bluesy, actually, because he injects a little improv in the middle of it by giving you another "And I said oh" about halfway through the chorus. Am I making any sense? Not likely. Let's just say it was quite cool!
"The question is can his voice hold up in the last show of this leg as it has the first few?"
Very good point--I was secretly glad to see the third show for fear of such a development; I hope the time off helps him! If not, there's always tea and honey (blech).