The classloader responsible for the ActiveMQ broker needs to be able to see the plugin class(es) as well. Right now that's not happening. I'm not sure how you've got your broker configured (e.g. as a module) so I can't advise further.
I integrated activemq like this: I copied the activemq_rar.rar in my jboss standalone/deployement repository. I generated an ironjacamar.xml file and injected it in the activemq_rar meta_inf repository.
I made some changes to the broker config file and the ra file in order to start my broker after deployement.
The order of deployment here doesn't really matter since the deployments have isolated classloaders. If you want your broker to be able to see the plugin classes then you should package your plugin classes with the broker (i.e. in the broker's RAR). Or you can deploy the broker as a module instead, create a module for your plugins, and then reference the plugin module from the broker module.