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I'm a little surprised about your mentioning needing to "discover" the other machines. Which may be related to this thread.

This is a bit of speculation, from my interpretation of your question I would do the following.

I would launch n nodes (anonymously if desired) on each machine. They would each publish on the same topic, not in their private namespace. And you could set the hostname parameter for each one in the launch file, or use the node to introspect the hostname in the code, and embed the hostname into the data.