Sure, my first thought is to do it in post

One of 2 ways will work fine here:

1. Render out your rain as a separate 3d element so you have more control over the luminance and color (chromanance) of the rain plate in your compositing package.

-OR-

2. Generate your rain directly IN your compositing package.
Most decent compositors have a particle generator that enables you to achieve rain, etc.
And if they do not, then usualy there are plugins avaiable that can help.

Good luck!