There are examples of cases where people would use Mr First Name, most notably with servants, for example, it is custom for servants to address children as Master John and Miss Jane, and I'm not sure, but wasn't Mr First Name common address by servants in the 50s, especially in the South? The issue with using surnames is that they are arguably more diverse than forenames which tend to run by fashion, given that the US was settled by a multitude of different nations that are inherited from different languages, whereas forenames are from a common language.