Work items are indeed the recommended way to communicate with external applications / tools (to avoid having to encode all this logic in script nodes inside your process). We have a work item implementation that might be useful, it all depends on what communication channels the application you are trying to invoke supports.
ExecWorkItemHandler allows you to call console commands using org.apache.commons.exec
https://github.com/krisv/jbpm/tree/master/jbpm-workitems/src/main/java/org/jbpm/process/workitem/exec
If this examples can be used out-of-the-box in your case, they should at least give you an idea on how to setup integration with your services using a custom handler. I could for example see a similar ant work item that allows you to call ant targets. We're hoping to extend the set of supported work item handlers for jBPM 5.1, with the help of the community ;)
Kris