If I remember correctly the particle system is a bit buggered at the minute - unless Simon fixed it for 4.2.5, but I'm sure he planned on fixing it for the new GUI version of the editor. I could be wrong though. Can you not add the rain as an animation temporarily for now? You could probably do it quite easy with a couple layers or so & move them down with Lua then snap them back to the start position. Rinse & repeat as needed. You could even create a few different frames to chuck a bit of dynamics into the mix.
& to anwer your question: no, not to my knowledge. The only current method as far as I'm aware is by controlling the visibility of an object. Speaking of opacity... what happens when you decrease the visibility value of a scene object with a particle linked to it?
game.CurrentScene.SceneObjects["test"].Visibility = 0
-- or if that doesn't work...
game.CurrentScene.SceneObjects["test"]:to(0, {Visibility = 0})
Let us know what happens as I don't remember if I've tested that out before as I don't bother much with the particle system.