Ultimately, org.netbeans.spi.project.ui.ProjectOpenedHook should be used. When a project opens, a FileChangeListener should start listening recursively from the project root downwards for the presence of .editorconfig files. No explicit Action should be needed, i.e., automatically the listening should begin and, if an .editorconfig file is found, it should be applied to the level where it applies.
The relevant properties are not many.
Just the above 8, apparently nothing more than that.