My feelings with respect to an upgrade of json-simple are mixed. The only thing 1.1.1 adds is a proper OSGi Manifest entry and a maven build. Unfortunately the pom.xml is broken and adds a plain dependency to junit (with default scoped). Also, debug information is switched on now, increasing the size of json-simple of about 50% (well, that's probably the smallest issue).
That was the reason, why I decided sometimes ago to avoid 1.1.1 and always embed json-simple where possible: It integrated in the jvm and OSGi agent jar and of course in the WAR of the web agent.
But more important, a version update won't fix AMQ-5091 I just tried the scenario you described with an json-simple 1.1.1 enabled JVM agent and get the same symptoms: When accessing the ActiveMQ included agent via port 8161 after an JVM agent has been attached, the
Unfortunately it seems that the project is quite dead (and if something happen, then very slowly. I tend to update to 1.1.1 (since I'm not so picky anymore about the size increase ;-), but again, that won't fix your issue.