Notice how the texturing is nicely aligned?
Seems to me that it would be possible to build a rough bounding volume tree with which you could determine which pages are potentially visible.
(Something I'm thinking about with my Deferred Virtual Texture Shading stuff)
Interestingly they're still reading back from the gpu which pages are visible at which LOD.
I wonder if that could be done more efficiently on the cpu avoiding the readback completely.