I've added support for custom plants in the GUI editor. One just need to st the custom plant directory and the plant contained within will appear in the context menu when you right-click your GUI in editor mode. You should be aware that inserting custom plants this way calls save on the source file. Although the usual UNDO commands will work if you wish to go back to a pre-plant state.
It works with multiple imports of the same plant, and multiple imports from different files. If you follow the convention of unique channel naming as demonstrated in this example everything should work fine across multiple imports. New betas are available now if anyone wants to give it a whirl.
Note: The properties panel is disabled for custom plants. And will remain so. I will add a plant export option soon that should make the creation of custom plants that bit easier.