Can't tell if this is exactly what you are after, but you can try it-
Shut down the server, then use 7-zip (or whatever tool you like) to open the jbpm-gwt-console-server.war\WEB-INF\classes\META-INF\CustomWorkItemhandlers.conf and add a line in like:
{code}
[
"Log": new org.jbpm.process.instance.impl.demo.SystemOutWorkItemHandler(),
"SampleTask" : new com.sample.SampleTaskWorkItemHandler()
]
{code}
Then jar up your SampleTaskWorkItemHandler class files and put them in the jbpm-gwt-console-server.war\WEB-INF\lib directory. Start up your server, then your SampleTask WorkItemHandler should be available to use without registering the handler from a ksession.
On a related note, I put in a jira to ask them to update their docs with this info, because it was not at all obvious how to do this. You can vote for it if you like: https://issues.jboss.org/browse/JBPM-3931