A plug-in doesn't let you specify the class of pieces of furniture that should be used, and even if you create a derived version of Sweet Home 3D, it won't be so easy to perform without changing code, because furniture instantiation is done at this moment in HomeController, ImportedFurnitureWizardController and FurnitureCatalogTransferHandler. Sincerely, this is something I'm not proud of, and instantiating furniture objects in only one method that could be overridden (like i did for Wall, Room and DimensionLine classes) has been on my TODO list for a while...

As for controllers and views you don't have access to them in plug-ins too, so you should probably better create a derived version of Sweet Home 3D. Creating such a version without modifying code source is completely possible, since it's the way I do it when I create customized versions of Sweet Home 3D for projects like this one. If you want more details, you know the place where you can ask questions... [;)]