Thanks for this plugin [:)]
It would be nice to program undo/redo. It's really not difficult to implement and you'll find plenty of examples in Sweet Home 3D source code, mainly in PlanController class. Once you retrieved the UndoableEditSupport instance with getUndoableEditSupport(), you just have to create an instance of AbstractUndoableEdit, overriding its undo, redo and getPresentationName methods, and finally call postEdit.