One thing that comes to my mind is that this might be caused by https://bugzilla.redhat.com/show_bug.cgi?id=754419. Basically, the agent code assumes that all the required properties in the configuration objects have values. This is taken care of by the GUI but in CLI, you need to specify a concrete value for every required property in your configuration object (+ others if you need to set them of course).
But that is just a guess. Do you see anything suspicious in either the server log or the agent log when you try to manually add the resource?
I have checked the agent and server logs which do not show anything related to the interaction, as a test I also just changed the parentresourceid to generate a semantic failure for which java as expected threw a run time error, this was not logged by the agent or server though so maybe cli interactions are not logged?
The configuration props i put in were the required props as defined by the rhq-plugin.xml for that type, I will try again as you suggest above with all properties added and see what the results are.