Hi, you can make what you want by determining what the camera view will see and load your meshes to suit that view only.
For example, travelling from island to island will involve covering an ocean with cloud layers possibly but all thats needed is a tiling ocean untill you actually get to see your target island, then apply a simple ent_morph for your lod stages or make your own lods and load them acording to vec_dist.

Once you take off you can then apply camera view to effect culling or poly clipping when models or terrains are outside of the camera view ro simply swap out for a low res file in place of a 'close up' high res file...

Your view over distance can be effected by carefully sculpting a suitable skybox scene mixed with an image of the view you want the player to see untill you need to add the models or terrains.
If you are not using the terrains for close up view then you dont really need them as they add a fair amount of wieght to the performance so swap out terrains for models when needed.

Hopefully Im on the right track here and perhaps offering usefull suggestions...