You're right, Richard, it might not be so bad doing the fields with objects, perhaps even a little crop and blend it in in PS. I'll make some flax and wheat...
Here's the near final iteration, reduced in size. I pulled off a nice 4800x2400 render in 8 hours at detail 6 and AA 0.6, GI 1/1/8. Now I must paint in a lot of fences, and some small stuff, and smoke from 'chimneys'.
It's really very rewarding to recreate something that has gone (as it is of course creating something that has never existed!). The nicest thing is to combine TG2 and modeling of things. But, boy, it takes a lot of time!!