Column names in Coda are some kind of identifiers, like variable name in programming language. It is not a good practice to change variable name dynamically, because other parts of the program may refer to it. Althought modern IDE allows you to rename variable in the entire program, this is still should be done before compilation or interpretation, by hand. Since Coda document is a program that re-interprets itself permanently, I think there is technically almost impossible to change column name from the inside.
And since you can change it only by hand, hardcoding method is pretty close to what you want in my opinion.
P.S. In your example on the screenshot column names are “A, B, C”, not “part1”, “part2”, “part3”. For the program these texts are just a content of cells in the first row. So maybe your problem solves with some architectural changes, when this parts shouldn’t be “Column names”, but should be values of some property.