I think I've finally figured it out.
I've been playing around with a 3D terrain/world rendering program called Terragen 2, which is designed for large-scale renders of outdoor scenes in photo-realistic quality, and can do movie clips as well by rendering individual video frames and then applying motion blur.
The program also has an OBJ loader which can load static meshes and move/rotate/etc them among animation frames, as well as applying lighting and transparency FX. It can do this not only as single objects, but also as procedurally-created populations which can be set to automatically sit on top of terrain.
Normally the population render is for trees, plants, vegetation, etc., but I've found it also works for buildings and other imported models.
AFAIK the free version makes renders compatible with GPL Open-Source projects, although the free edition doesn't do animation (only one single frame at a time) and has other limitations.
Here's a quick example with a simple, spartan terrain and no clouds/sky stuff, but just to show off a 3D model of a building in a population. I got this "factory" model from an open art collection, according to that site this model is public domain IIRC.
...So all I need now is a few good OBJ models of modern buildings of various types, even half a dozen is good, and I can make what looks like a city.
I'm in the process of getting the full version of the software with animation.