This appears to be a feature of the past. I've discovered that the DataType implementations were handled in the rule flow <parameter> element in addtion to the work item definition. Currently, there isn't an element or attribute in the BPMN2 XML to set such. Additionally, I've discoverd that the IDE in eclipse barks if you attempt to update any definition that is not StringDataType in the work item definition.
If I were designing this....
I'd make a single point of definition for these handler parameter types and import them on compile of the processes definition. It would be wonderful to bring this feature back in any stretch of the imagination!