It would be doable IMO. Just use a lot of sprites when your zoomed out. (You can render a 3D highly detailed object then take pictures of it on a "green screen" using a script while it performs its animations. Now you have a folder full of nice looking sprites with full animations and from a distance they would look 3D. When you zoom in just switch over to the 3d models.

If I didn't describe it clearly look up how games like Diablo II were made as they use a technique similar to what I described.