The connector-service resource has only a factory-class attribute and does not specify from which module the class should be loaded.

        <connector-service name="b"
                           factory-class="bar.foo">
            <param name="foo"
                   value="${connector.value:bar}"/>
            <param name="bar"
                   value="2"/>
        </connector-service>

This means that the module defining the class must be a dependency of the org.apache.activemq.artemis module since it's Artemis that currently loads it.

A module attribute should be added so that the class could be loaded from any module.
The messaging-activemq subsystem would then be able to load it and pass it to Artemis's ServiceRegistry (similarly to the interceptors).