Thanks for opening this issue, Dean! Would you be interested in helping out with implementing this feature? We'd need to define a way for specifying more than one host, port etc.

One way to do it would be to provide a global option for our pluggable option system (https://docs.jboss.org/hibernate/ogm/4.1/reference/en-US/html_single/#_configuring_mongodb) which would allow to configure it similar to this:

public class MyOptionConfigurator extends OptionConfigurator {

    @Override
    public void configure(Configurable configurable) {
        configurable.configureOptionsFor( MongoDB.class )
            .replicaSet()
                .replicate( "host1", 27017 )
                .replicate( "host2", 27017 )
                .replicate( "host3", 27017 );
    }
}

But having a configuration value for persistence.xml would surely would be a good thing as well.