I don't have Carrara, but in RayDream it's really quite easy. There's a tab in the properties window for all lights called "Gel." It gives you a few options like blinds and gradients and also allows you to put a map in (or even a movie for animations). You could create a mottled texture in photoshop or some other image editor (maybe try a sponge filter or something). If you're just trying to make the light look like it's streaming through a tree, make your gel black and white, otherwise, go for some color, too. But to stay photoreal, be careful about too much color on the backlight; it's easy to suddenly look like you're in a 1960's head film. - Chip