Currently, the org.apache.activemq.gbean.BrokerServiceGBeanImpl GBean is configured in the activemq-broker config and is thus built into a serialized configuration during the build process. Looking at the plan.xml, there is a commented-out section that specifies an external configuration file to be used. Enabling this section allows for the broker to be configured by an external file.
This configuration should be changed so that the activemq-broker configuration refers to an external file by default, rather than being internally configured in the plan. This external file can exist somewhere in the geronimo folder hieracy (e.g. var/config/activemq.xml). This external file should contain the same minimal configuration that is currently specified in the GBean descriptor in the plan.xml.
The benefit of having an external file by default is that the activemq broker can be configured without having to rebuild Geronimo. This makes the ActiveMQ configuration much easier.