There’s not any reliable way to do this with the current vertex shader. In general, we tend to get precision / jittering problems when zoomed in close anytime we try to work with geographic coordinates in shaders, so we avoid it. But if that’s not a problem in your case, you can easily change CentralBodySurface to pass the geographic coordinates of the southwest corner of the terrain tile to the shader as a uniform. Then, add the result of the mix line you mentioned below to that southwest corner to compute the geographic position of the current vertex.
If you can tell me a little more about what you plan to do with the geographic coordinates, I may be able to recommend a more robust solution. If you’ll allow me a shameless plug, Patrick Cozzi and I wrote a book, 3D Engine Design for Virtual Globes, that has a couple of chapters with a pretty good discussion of some of the problems (and solutions) that come with rendering planet-sized scenes on single-precision GPUs.
AGI weathered the hurricane without too much incident, though lots of folks still don’t have power. Are you located anywhere near the east coast?