Because all I need to do is run a stored procedure and pass in the instance id. I don't need to bind to any fields on the existing form, and thus, have no need for my stored procedure to return a "text" or "value" value to assign to anything.
What I ended up doing was renaming Publish workflow action to "Duplicate" and making this only visible in the stage I want to duplicate and when this is clicked to run the stored procedure. I changed Publish to Duplicate in all the relevant resource files. The icon appears on the bottom with the other work flow icons. This makes it more intuitive as well.
Would this pose an issue for upgrading to a new version - the fact that I renamed Publish to something else? The only place I wasn't sure where to make changes was to where the Publish text is saved for the workflow grid on the edit workflow page. My drop down list shows "Duplicate" but my grid of saved workflows still shows Publish.