OK, got it.
I can now control the fog distance through objecttypes.tbl, however not for everything - I still can't control the fog distance for asteroids. Also, as far as I can tell, explosions, trails and things like that are not affected by fog at all, which breaks the illusion a bit, and the objects still pop out once they reach the maximum fog distance.
In the wiki example there is
+Disappear factor: line which looks suspiciously like the thing that I need, but it gives an error if I try to include it into the objecttypes.tbl.
Hope these are not silly questions, but I've been at this just for a few days now, and am not a coder but an artist anyway.
As for the range option in fred, that's just for the radar range, isn't it? Or am I looking at the wrong one?
In case anyone's interested, here's what I have for now:
https://www.youtube.com/watch?v=av-GXBqNBpoLooks much better ingame - video compression destroys a lot of detail.