Exclude files from nested jar

We’re producing standalone jars whose dependencies are nested as jars (produced with https://github.com/robyf/gradle-jarinjar-plugin), like this:

|- library1.jar
|- library2.jar
|- com/standalone/project/** <- classes

We would like to exclude some files from ‘library1.jar’. How would that proceed ? I tried various exclude patterns within the jar task of project com.standalone.project, but to no avail. Any help appreciated.