is it possible to cut one 512x512 square of LOD terrain out of a group after it's been joined with them? like take a cursor or something and select one square of one and hit separate or something?
If you detach and delete it, it is gone. What your thinking of is using delete in the facet editor (Shift F). Deleting the little triangle is not permanent because you can still see them when you click on LOD Terrain > Color Surface Flags and remove the delete flag.