Comment 4 for bug 1248632

The relevant code in Cordova Ubuntu is this

src/plugins/fileapi.cpp
m_persistentDir(QDir::homePath() + "/.local/share/cordova-ubuntu/persistent/" + QString(cordova->get_app_dir().toUtf8().toBase64()))

src/cordova.cpp
QString Cordova::get_app_dir() {
    return m_www.absolutePath();
}

So, in theory, doing something like base64.b64encode(absolute_path_dir) should work.