I've been using the Terrain Grid System (which is incredible and has saved me so much time!) up to this point with Terrain game objects and haven't had any problems selecting cells in the editor. However, I recently started working on an indoor scene and, instead of using a Terrain game object, used a root game object containing many floor tile meshes as the "terrain" for the grid. The grid generated as expected, but I noticed that I was unable to select cells in the grid that weren't on top of something with a mesh.
Here's a screenshot of what I mean:

You can see the blue border around all the floor tiles that are under the root game object. If I click any of the cells inside the blue outline, things work as expected, but if I try to select any that fall outside of the blue outline, nothing happens.
For some background, how I'm using the system is overlaying a single grid over the level and then marking individual cells invisible to customize the look of the grid. In this case, I'd like to select all the cells that fall outside the map and mark them invisible, but I'm unable to select them.
It seems to be that a click event isn't registering unless there's a mesh under the cell? I say that because if I add a plane game object under the root to effectively make the area a square so that all cells are on a mesh, then I can select them.
Is there a simpler workaround for this other than what I'm doing? If not, I can make do!