[Knokke]

This is not really working unfortunately, take any object,say first object in fmb, open the mat file, the distance is set there anyway to 10000. Now go in game and measure the pop up distance for that object (I did that long time ago). The pop up will be some 3000 meters for first object with normal view if I remember correctly. The pop up distance is somewhere else, and I suspect it's in the .msh file... The houses having the greatest visibility range are the Norway ones, about 7000m (not sure though about the Slovakia castles and churches they might be even more "visible").

The visibility parameters for various objects LODs are indeed stored in the *.msh files. I didn't check if the *.msh files in Il-2 are binary or text files, but it should be easy to open them and convert to msh/text format with MshConverter for editing the LOD view distances.
At the moment, I'm creating some static objects for Theatre if War (a game based on the Il-2 engine) that could be used on your project and serve as visual landmarks. The bridge at Grave in Netherland being one of them (one of the Market Garden bridge).
Here it is in ToW:
http://pigeonh.free.fr/grave-6.jpg
Let me know if you'd like to use it.
-Knokke