ActiveMQ supports pluggable JAAS modules that handle the authentication of incoming requests. ActiveMQ comes preloaded with a few JAAS modules, none of which supports MongoDB as the backend repository of the authentication data.
OpenDJ is a powerful LDAP server based on the popular OpenDS server. OpenDJ supports multi-master replication, failure auto-detection and recovery. The replication setup is extensively documented and fairly simple to setup. Apache Karaf is a customizable OSGi container with excellent support for application deployment. For OSGi application requiring an LDAP server, it would be ideal to be able to deploy OpenDJ as just another feature of the main application in Karaf.