We have to do something similar on every project submitted to local government for developmental/siting assessment.
The existing and proposed shadows must display differently, so either different colour or texture, for existing and proposed/additional shadows cast.
We have experimented with saving shadows as image and export to DWG, overlay in Illustrator or Photoshop and import back to Revit, but the quality less than perfect and has to be redone if the building changes at all.
We resorted to setting a view of the existing buildings, tracing the shadows with filled regions (with transparency and colour), then switch phase to proposed and the new shadows overlay the existing (usually takes a few hours to do, but quality is great).
If Revit was able to do it, it should be a relatively straight forward concept to implement, and would save us hours and hours on every project, simply by switching a few sun settings.