The gn template brave_xml_preprocessor can be used to process XML resources files before it is compiled.

A target brave_java_xml_preprocess_resources has already been set up. To add a new XML resource file for pre-processing,

1. Add the XML file path to brave_java_preprocess_xml_sources in android/brave_java_resources.gni.
2. Add a python file to where you want, and add the path to the python file to brave_java_preprocess_module_sources in android/brave_java_resources.gni.
3. In the newly added python file, define a function _ProcessXML(root) which takes one single argument root as the root element of the XML root etree Element. The function shall return the processed root Element.
   1. You can do whatever processing you want with the Element. For example, insert a new node by parsing a string using fromstring(text) and insert it at a given position with insert(index, subelement).
   2. Remember to register namespaces. You will need to at least register the android ns by doing

      ns = { 'android' : 'http://schemas.android.com/apk/res/android' }
      for prefix, uri in ns.items():
        ET.register_namespace(prefix, uri)
      

      as well as passing namespaces=ns to all find(match) calls.
4. You can then use the processed resource with a brave_ prefix. For example, R.main_menu will now become R.brave_main_menu.

See this branch for a simple example for what you can do.