SOurce SDK : Pouring Rain

I don't believe there is any other method to add ran than what you have already mentioned.

even though the precipitation only adds rain, you could add fog and atmosphere yourself with different entities, although im not sure whether or not you can restrict fog to only be outside.

What happened to that nice rain in the ep2 graphics show case map?

That nice rain is a generated model, that only can collide with dev texture floors, which is more fail then AIDS.