Is it possible to set up mirrored queues on different broker instances, residing on different machines? If so, how?
If not, what component(s) might I need to subclass/override to do something homegrown?
Not really sure whay your incentive is for this setup.
If you have multiple brokers in a network, you can't really tell which machine a certain message is on (since it's forwarded to the broker where there is a connected consumer).
If all you need is wiretapping to a totally different ActiveMQ server, you can use Apache Camel for that (it's included in ActiveMQ).
You just need to configure a connector to each ActiveMQ instance.
Actually, in a response to another question, I wrote a small Camel route that copies messages from every queue starting with FOO. to another queue with the name COPY.<queue name>. It might be useful in this case too.