As I recall, the mod "Homefront" implemented falling snow for BF:1942, as did DCX. Even though some of the code was removed from the BF engine for BF:2, it might still be possible. I'm sure it's at least looking into.
As I posted in the other WeatherFX thread, it's totally possible. I believe it was done for BF42 using particle effects. No lag, but the falling precipitation would be visible inside of buildings.