I started up firebug to debug it and I can see an error when the desginer tries to load a BPMN2 diagram when I have my custom Work Item Definition.
Here is what the firebug output looks like when the designer loads up fine (without any custom work item definitions)
Here is what the firebug output looks like when the designer gets stuck in the "loading" screen (when I have a custom work item definition)
I solved the issue.
The designer gets stuck when your work item definiton does not define an icon.
After adding an url to an image in the "icon" parameter for the work item definition the designer loads and I can use my domain specific service.
Hi Andrew, I was just trying something similar in Eclipse, instead of Process Designer and I found that the problem isn't the icon parameter. I took the icon parameters out and custom service tasks appeared. However, I found that the bpmn graphic editor would only show updated work items after I ran the process. It's as if running the process loads in the new work items.