[level]

The article covers the use-case of making small modifications to the build. react-app-rewired[1] and customize-cra[2] give you access to tweak the webpack build indirectly. While babel-macros[3] in CRA 2.X give you access to the babel transpilation step without having to eject. While it's not exactly what you're referring to, it does get you a little bit closer to the build.

[1] https://github.com/timarney/react-app-rewired/

[2] https://github.com/arackaf/customize-cra

[3] https://babeljs.io/blog/2017/09/11/zero-config-with-babel-ma...