I do not think sheet metal shapes would do a very good job of mapping the wear on a lathe bed that is measured in thousandths. But, Russ is a pro, and I could be wrong. Ken's setup was an experiment designed for grinding a lathe bed with a normally handheld grinder mounted to a trolley, which was also used to map the existing wear and the progress.
Bed ways become swaybacked typically from use, with most work done near the chuck and therefore more wear in that area where the saddle typically moves back and forth a lot. The ways also wear sideways from side loading from tool to work loading. Before we start removing metal, we need a map of where things are at the start so we can make plans to remove the minimum amount of metal to get the desired results. We also need to test the progress of the work as it progresses, and finally we need to be able to know when we have accomplished the repairs to within the required tolerances. We make jigs and templates to assist with those measurements, which cannot be done with a ruler or other simple everyday tools.
In earlier times, lathes were expected to be hand scraped, that made them "professional", like the big guys used. Of course, expecting a scraping job on a $300 lathe was ridiculous, even in those days, so they just put some random scraping marks on it after it was planed, milled, ground, or otherwise manufactured using machine tools. It was scraping, and it was done by hand, so the lathe was advertised as "Hand Scraped" in the promotional material. A lot of machines are "reconditioned" today by people with scrapers who make them look nice and pretty. They also put a nice coat of paint on the machine. It is still a worn out POS... Of course, some of the best machinery in the world is also finished by scraping, carefully and to very tight tolerances. A "reconditioned" $2000 Bridgeport series 1 is not one of the great ones... Caveat Emptor!
Edit: People who have the skills to do that work also have the skills to know the difference between fine work and a con job...