Program "project" structure - how to use?

[komame]

In the new program/project structure, you can have additional files with program code, graphic data (images), and binary assets. Does anyone know how to use it? Can such data only be read, or maybe is it possible to modify it as well (for binary assets)?
Tim mentioned such a possibility, for example, here: https://www.hpmuseum.org/forum/thread-19...#pid169882 and here: https://www.hpmuseum.org/forum/thread-16...#pid148289 but I can't find a single example anywhere. The only thing that worked for me was an additional file with source code (PPL/Python), but I can't reference the other types of files.
Tim?