Can OmmConsumerConfig::username be specified in a EmaConfig.xml (perhaps in Consumer section)?
No, username cannot be passed from XML config file.
It is usually set from code:
consumer = EmaFactory.createOmmConsumer(config.host("INFRA_HOST:14002").username("MY_USER"));
and can be optionally passed to the application as one of the command line arguments,
or by default, it's extracted from the environment.