Great effort Ulco! I'm really glad someone with a good grasp of TG2 is trying this.
I love Luminism, so I hope you won't mind me sharing some ideas with you. You can take or leave them as you see fit. If you all ready haven't thought of it.
I thought about this for a long time but always ran into a lack of technical knowledge at on point or another so I did not do it my self, but perhaps you will pull it off.
The first thing I thought to do was, as you say, random fractals. That was pointless I found. But you are getting much better results than I did so far, so who knows.
Then I thought a satellite resource for an image map to get the first part of the image. The back ground is not real, so I think that random fractals would be fine.
Anyway, what would be the best file type for the image map of that location? DEM, or something? This is one of the technical problems I ran into. There are no tuts I know of on using satellite data as image maps. BUT THERE SHOULD BE!!! If anyone cares.
So assuming you get a landscape your really happy with, that reflects luminist scale. Then Lighting is the most important part.
I was thinking you could use a large flat rectangle, make it reflective, and use that as a bounce light. That should help get light where you need it. Also Lumnisits treated landscapes like portraits, so it would be appropriate to bounce light.
If you could use some of those techniques people posted, where the land produces light, that would help to. The trick would be to get everything in play at once, while keeping wanted shadows just above black.
There was also a post by mood flow where he used an image of clouds to light the shot. I couldn't make sense of that one, but maybe you can. At any rate, the landscape is secondary to the light. The light is everything.
Good luck man!
*Oh yeah, I also thought it could be smart to do three renders. One to light the background, one to light the mid, and one to light the foreground, then photoshop them. But you could also try a merge images in photomatix; no idea if that would work though.