"Yes, it is likely to be little more than a blob at that scale"
You could, at the far-out zoom levels, just make it a blob. Several visualizations I've seen of social networks and music turn subgraphs that would be too confusing to display into a colored blob, the size of which is proportional to the size of the subgraph. I'm assuming how they function is they just blob-ify everything more than X distance from the node you are looking at.
Just food for thought, I think your idea of a sort of Google Earth type of display is great.