Well, there is already a snow script - it's included in Kegetys Winter Nogojev island. I'll extract it for you when I get back, at least if no-one else has. You could also do it yourself, just get PBO decrypter 1.5 from ofp.info's Utilities section and just decompile the Winter Nogojev addon.
Also, I was wondering - Since the rain-drop effect appears inside houses, too, wouldn't it be possible to check if the 2d position for the rain drop contains any objects? To give a poor example -
_x = (getpos _unit select 0)
_y = (getpos _unit select 1)
[_x + random 10 - random 10,_y + random 10 - random 10,10]
? nearestObject... ---- but then again, that wouldn't work... But if you could just check of if the position in question has any object(s) at its position...
Once again, just thinking out loud...