I'm having a similar problem - we have an application that is joining dynamic JSON data to a map service by a unique ID shared between the JSON dataset and the map service... everything works brilliantly in our application, although we ended-up creating a custom breaks function; however, when we "print" the map using the default template, the LegendLayer passed to the server ends-up overriding the break labels rendering in the application with "override 1", "override 2", etc., like what is seen in Daniel's attachment (exOverride.pdf).
The resolution, it seems from John Gravois' previous response, is to ensure the map service contains the field/data... unfortunately, this isn't an option for us. Has anyone had success with a workaround? My initial thought was to create a custom template where we could send an HTML object from the client-side (the object being the legend) - I haven't come across supporting documentation yet to pursue this... we are currently using the default print template.
Thanks!