Great footage of California 1972! Never seen film of the mirror belt before.
I'm sure the reason he did not immediately put on the guitar was nothing to do with the fact that he was wearing the mirror belt. It was more likely to have been some sort of strap malfunction or something Charlie did wrong. Anyone who has seen one of Elvis' guitars up close as I have, cannot fail to see the huge amount of scratches all over the front of them- so it's unlikely he would have been too concerned about his belt scratching the back of it.
He did, after all, put the guitar on at the instrumental break.
Nice pic here of the belt in question: