I've recently been trying to dig up any technical information on how exactly the "Strategic Zoom" is implemented in Supreme Commander.
I'm guessing that the game uses a layer-of-detail system with a large amount of different layers, but one developer quote I keep encountering is "it would be very hard to implement this in [Other game] because they haven't designed it from the ground up with that in mind."
Scaling the terrain effectively would be the biggest challenge, I think. Even at the farthest zoom level, the map still looks intricately detailed. This thread mentions that the maps in SUPCOM use Megatexturing.
I don't know the exact details on how they did it, but I have a pretty good idea and its similar to what you mentioned with "layer of detail", though it's usually called "level of detail" (LOD). It's probably similar to the way mip mapping works, but with models instead of sprites. As you zoom farther and farther out from the surface, the model level of detail decreases. Eventually, the models get so small that you can't see them anymore and symbols would be more beneficial from the UI perspective.
I imagine that the same technique could be used for the terrain maps. The map data could contain multiple layers of terrain detail generated in the map editor. At the highest resolution, you're able to see individual rocks, pits, grooves, etc. and it is a very high vertex representation. As you zoom further out, rendering those millions of verts would become GPU intensive, so you'd switch the terrain to a slightly lower poly version which renders faster but with little or no noticable decrease in quality. The terrain in SupCom isn't deformable and the light source is constant, so as a further optimization, you could even bake the shadows into the terrain texture rather than calculating them each frame. Then, the only shadows you'd have to calculate are unit and cloud shadows depending in your LOD.