Cliff will probably reply with a more correct solution, but since you are frustrated and confused I thought I would offer my solution to your problem.

I don't think there are any 'rules' against proxies communicating and sharing data, so in your ModelPrepCommand initialize only the XML data proxy. When it has finished loading and parsing the XML send a notification to start another command that adds the other proxies to the system. These proxies can then grab instances of your XML data proxy and go about their business.