I'm not sure about the usage_template approach - seems like it might be hard to make it work, while still supporting formatter_class. (Though maybe it's not so bad since the formatter class methods are all considered implementation details.) I'm open to patches though if you're willing to provide one.

In the meantime, a simple approach that will work is to override format_help():

class MyArgumentParser(argparse.ArgumentParser):
    def format_help(self):
        help = super(MyArgumentParser, self).format_help()
        return headline + '\n' + help