I tried to picture how that would work. I don't see caustics working in the area of the mud cloud but outside that area why not? Using the right filter(gel) with your lightsource would give you the caustics. It's been over a year since I tried this and I used something I believe was called "caustics generator" to build the gel. It worked well as I recall, but I got carried away with the effect and it looked a little gaudy - in my defense I was in a hurry.