With geotagging of photos, we can now pinpoint where we were at the time a photograph was taken.
Since GPS satellites also provide time information, we should be able to also know precisely when it was taken.
That allows us to accurately determine where in the sky the sun is relative to the photographer and the subject (landscape.)
If there is sunlight in the image, along with shadows, then it should be possible to determine what objects are of what height and thus fill in the complete 3D map in more detail than is available simply with a terrain map from the likes of Google.
If we put all of the above together, then we should be able to take any given landscape image with sunlight and then move the location of the sun and have an updated image fall out of the process. It may also be a small step from that point to simulate the day of the year.
If it was possible using a plugin or otherwise "change the sun position" that then allowed you to selected where on the sphere to place the sun, is that going too far in post processing?