Thanks for the reply Brian.
Maybe it would be usefull to add the following to the documentation. I couldn't find this anyware. It might help others.
If you want to read the operation description of a child resource you have to navigate to the child node by using the key-value pair "child name" , "*"
E.g. if you want to read the operation description of the 'add' operation on jms-topics, you'll need to build the address like this.
ModelNode address = op.get("address");
address.add("subsystem", "messaging");
address.add("jms-topic", "*");
...