That is correct, the RPC trees or RPC objects from Revit are not supported in Showcase. But you can easily download trees from Autodesk Seek and used them in Showcase. If you have 3ds Max Design, the possibilities are even wider as you can export objects as FBX. You can easily find really low polygon trees on Google Warehouse, open them in 3ds Max Design and Export them as FBX.
You can open your Revit model directly in 3ds Max Design to facilitate the placement of trees. I would still suggest to keep the workflow from the main building = Revit to Showcase and for the accessories only (Select only the accesories from the 3ds Max Design scene and export selected as, FBX) 3ds Max Design to Showcase.
Hope this helps
Marion