No, there is no variable for rain that the scenery can use.
I added one in the ActiGate module, so by using that module it would be possible. But personally I would only depend on such a module users have to install if it is a really import feature of the scenery.
The "rain" functionality is associated with controlling reflection, within the model material settings.
The checkbox is "take precipitation into account", and you can apply an offset, which determines the time between rain starting and the material setting altering.
Basically, this just switches on the reflection map, when it is raining, which in combination with a bump map (to break the reflection up) is what is used on model surfaces to simulate water on aprons / roof etc.
@Patrick: this functionality is not available in FS9, BUT all of the MDLTweaker tweaked models work in FSX (with the minor exception of transparency, which will display but is faulty). It is certainly not a reason to develop FS9 exclusively!
The only reason I could think of developing for FS9 is not wanting to learn the new tools!
So you can have tweaked FS9 models, FS8 groundpolys, and all of the increased FSX texture resolution and material settings! That's why I don't develop for FS9!