hquplugins.org will have some HQU documentation.
It would be great to augment the LiveExec plugin to issue JMX queries. What types of resources do you want to attach to? The LiveExec view is currently only attached to Platforms, but you could also attach it to any servers or services that have JMX configuration. Views are setup via live_exec/Plugin.groovy
In order to issue a JMX call originating at the agent, you will need to use the LiveData subsystem.
(See LiveDataManagerEJBImpl.java). The plugin which does the work is MxLiveDataPlugin.java)
Most of the functionality that you will need, however, lies in ResourceCategory.groovy in HQU. It adds methods to Resource (org.hyperic.hq.authz....Resource) objects which are a little easier to use.