Not a suprise, many of these guys grew up in the country and played football and baseball as kids. Tim McGraw played in college (he didn't know his dad was Tug McGraw until he was a lot older and his dad didn't do anything to advance his baseball education). Apparently Larry the Cable Guy was a HS star as well.
Charley Pride played in the Negro Leagues in the 1950s. He actually did play some in the minor leagues in the 60s.
I'm not sure if it was more of a publicity stunt or not but Garth Brooks actually was invited to several spring trainings. I believe it was more for publicity than anything else but he seemed to do ok, for a guy in his late 30s.