Pilch, there's a solution for your case with SH3F files since version 2.2, but this is not easy to use until I release a furniture library editor.
You should create a SH3F file that contains your pieces (you can create it from the one accompanying this message or from the furniture you imported with ExportToSH3F plug-in), edit its PluginFurnitureCatalog.properties file and add a property named planIcon# with a value equal to the path of the icon that will appear in plan.