We shall fight in the dark.

[Gandhi]

I'm thinking something like this:

1) Draw all static geometry (trees, walls, ground etc)
2) Run a postprocessing shader that desaturates all colors by some amount
3) Draw all player and vehicle geometry
4) Run a postprocessing shader that desaturates quite a bit less to keep color recognition
5) Draw all weapon effects to a separate framebuffer
6) Run a standard Bloom postprocessing shader on that to give weapons some glow
7) Put it all together and push it to the screen

This has the effect of putting a much bigger emphasis on weapons fire at night time, while the desaturation mimics how our eyes work in low light. Ideally weapons fire would behave like small light sources, but I have no idea if the engine could handle that.

It would turn into a totally different game at night. Aircraft would become much more stealthy, you'd probably hear a vehicle before you see it and a big battle would be one hell of a light show. Infantry could be pretty stealthy too if they hold their fire until they're in position. Different challenges. I think that's more fun than "Oh look it's night, better turn on my night vision goggles".

Just a quick and dirty mockup to give you an idea of how this would look. It would probably be quite a bit darker ingame, and obviously you'd have to do something about people just turning up their gamma settings.