Minor Description
The Java Broker currently has code to allow for using Derby as an in-process store. It is relatively trivial to modify this code to allow for the use of any JDBC compliant database as an external store provider (though performance is unlikely to be impressive). The advantage of using an external JDBC store is the ability for third party libraries to easily inspect the store, and for storage to be provided by existing infrastructure.
Some tweaking of the store is necessary for those parts which create the tables since SQL92 datatypes are not universally implemented.