I managed to make some progress on this issue. Initially I updated the test code to allow the actual test method to install the bundle under test - instead of supplying it to the paxexam-karaf container as an "option". This seemed to resolve the issue I was seeing with the bundle not being able to connect to the broker. The test itself, however, still failed to connect.
I updated the connection URI (of both bundle under test, and test probe) to use "vm://default", as specified in the activemq-broker.xml configuration for servicemix's embedded broker. The test has since worked consistantly. Given the aforementioned activemq-broker.xml file also contains the following entry, shouldn't "tcp://localhost:61616" also work?