how about trigger nodes and not loading model by model but groups or streetblocks? whenever you are near one "checkpoint" the surrounding modelgroups are loaded and unloaded. So you would only have to scan for the nodes.


the only thing i would be worried about are the huge texture sizes you have and the ammount of space and time they will need to load.
From seeing google earth a method could be to load the models with very small placeholder skins and once the loading is done run a rutine to replace the small skins with bigger ones one by one. So you wont get lags or framedrops because of huge buldge loadings.

just an idea though

cheers