Java Native Library Task in flat-folder multi-project

I am still very gradle noob so basic principles are eluding me. After I discovered there was nothing in the supplied framework to handle native library declarations in java, I read the docs and forums and found code to do it. I then abstracted a method, resulting in:

def nativeLibraryTask(File nativeSubDir) {
	logger.info('nativeLibraryTask: nativeSubDir={} exists={}', nativeSubDir, nativeSubDir.exists())
	def parentDir = nativeSubDir.getParent()

	repositories {
		flatDir {
			dirs parentDir
		}
	}
	dependencies {
		implementation files(nativeSubDir)
	}
	eclipse {
		classpath {
			file {
				whenMerged {
					def nativeLib = entries.find { it.path.contains nativeSubDir.getName() }
					nativeLib.nativeLibraryLocation = nativeSubDir
				}
			}
		}
	}
}
//todo this works but only when the nativeLibraryTask is defined in the same file
dependencies {
	//todo using project.rootDir.parent because I could not make relative paths between projects work
	nativeLibraryTask(new File(project.rootDir.parent , '<other-project>/<native-lib>'))
}

I would now like to move this method to the master build.gradle so it is available to all sibling projects. However when I do that, the classpath reference is not created. I presume this is a relative file reference issue but have not figured it out yet.

Correction: Logging tells me that the relative file reference is working (the file exists). So here I would like to know how to create a relative path more elegantly using $rootDir or the project reference. But my bigger problem is that the code inside the method is not creating the classpath reference.