Yes, I've also got guitar strings and curtain strings (cables?) in varying diameters with wooden handles, but I've always struggled with burned grooves on faceplate work. I made myself a tool from a piece of aluminium with a rounded nose, mounted in a wooden handle, but this has to almost go to the melting point of the metal before it burns the wood.
Now, whilst the trick with the burnt line is certainly a good one, I am not quite sure how Ed could have used it on the third bowl, as the burnt line would have to be right on the edge, essentially taking the edge away.