L4D2 had some quite big changes in the engine. Thats also a reason why it runs alot smoother than tf2. Basicly as all downsides slowly get removed. Also, in l4d2 its for all players and regardless of the area.If I remember, L4D2 did this too though, or at least COULD do this, but it was a bit different than how the original poster described it
This should be something we could ask valve to do. They made it so we could create our own "Weather" particles now, so why not this?