Source for this? Searching for "Hardest working in show business" without quotes turns up something relating to an entertainer, and searching it with quotes also returns a bunch of entertainment-related matches.
I'm a native speaker that grew up and lives in the U.S. People use it all the time. I don't have any scientific research papers available to show you, but I promise you - this is a common expression.
American culture for the past 40 years. Seriously, this is a common idiom.