I don't know when the ReferenceTarget decoration ever was need (or why others reported it as an obstacle), but it is not needed in 2013.
And, while the code is generic, there is no need to establish a listener -- the assembly has to be registered, but no "plugin step" need be established. So I put the answer to my second question as, there is direct communication between the UI developed
workflow and the assembly and no need to "publish" it to any entity.