The only way I can get the effect I want is to have an attached ODF light simulate daylight when it is otherwise dark in my map (which I can then use KillObject() to get rid of when needed). Problem is, I haven't been able to get this type of light to either affect terrain or show up at all without the cone or halo values. Just to be clear, my goal is to have one (or maybe more, but hopefully few) lights attached to an object (geo_prop_splash, I believe) that brighten both the terrain and objects as though it were daylight, meaning no cone effects or anything extra - just want everything brightened. I tried setting omni - radius to 1000 and setting the cone paramiters to 0, and it did nothing. There was no light at all, when before there was at least a cone. I have a sneaking dread that it's impossible to change the terrain brightness except in ZE, but I hope I'm wrong.

Terrain brighnes is not affected by dynamic lights, as I understand it.

Yep, terrain lighting is burnt-in using ZeroEdit. Only objects are affected by lights in-game.

Thanks Tean and t551. Ah, just thought of something. Can you call a different world file at the start of the LUA? That way you could have two different ZE files for different gamemodes....

That would be kinda pointless. Two world files would create difficulties that would be better to have just avoided I would think. You could just go into the ZE, and when you get into the world, just create it in one layer, like conquest, put everything into that layer. Then you could change layers, to say, assault, and create a world in that one. Since objects put in one layer only appear in that layer during gameplay, anything put in the conquest will not show up in assault, and vice versa. I think, to get rid of the clutter of stuff that would show up in ZE, when going to edit the assault, don't load the conquest layer. This would of course make it very large sized, but so would two .wld files, each with it's own layers.
I don't know if this works or not, I haven't tested it myself, but it should work for whatever you are thinking of.

Unfortunately, it won't. As Tean pointed out, terrain brightness is burned in using ZeroEditor, which means once you've set your lighting up, that's it - no changing it. In fact, I tried this earlier, and the lights could not stay simply in a layer - it was universal. In order to produce two separate lighting effects for the terrain, you would need two world files. I'm just not sure how to set it up so that different gamemodes call up either the day or the night wld file, depending.