You are perfectly free to access the data from any Proxy inside any Mediator by simply retrieving it and using it as you describe.
Keep in mind that the Mediator doesn't become a final repository for your data, even though they often cache a reference to the Proxies they need to talk to frequently. So the super-mediator idea doesn't really make sense.
However, its perfectly reasonable to consider a Proxy that takes data from other Proxies when called and assembles it into a composite structure if need be. This might be useful for, say, populating a tree control in the view where the data structure needs to be composed of data retrieved from disparate services.
The only problem then is the proxy that made the composite may need to communicate changes to the structure back to the source Proxies for saving back to their services.