It is not usable in long running processes like IDEs, when you need to configure and change it on fly.
I had to work around it by:
try {
Field include_path = ImportedFiles.class.getDeclaredField("INCLUDE_PATH");
include_path.setAccessible(true);
List<File> o = (List<File>) include_path.get(null);
o.clear();
o.addAll(FileSystem.getPath("plantuml.include.path", true));
} catch (Exception e) {
LOG.debug(e);
}