the usage of the center is hardcoded in the RotationService and altering the RenderTransformOrigin will not have much effect. So, for now there is no way you can change this.
The good news is that we'll release a new version of RadDiagram in a couple of weeks (Q1 2013, around Feb 15) and we have made it possible to plug into the core services, including the RotationService. This means that you will be able to alter where the pivot point of the rotation is.
A visual way of altering the rotation origin is not something we plan to do, but you can always speak out through the various channels (PITS in particular
Hope this helps, Fr.