Tim, you can use String as content type it just requires to be configures as string type in the item definition. Once you have custom type defined you need to either use that object directly when providing result to your work item or provide xml representation as string so XStream can build the object for you. If you use java.lang.Object as structureRef it will use it as object without any type checks or transformations.
Let me know if there are any other concerns in this.
HTH