unless i'm mistaken, caustics is the reflected light onto the floor, in the above pic?
and the wavy light at the bottom of a pool, etc?

what i'm after, is the reflection of the object, the part you see in the larger cube
easiest thing to do, is after the reflection map gets rendered, is multiply that times itself

think of a wood floor, you can see a reflection is newly polished ones
the light on an object bounces off the floor and into your eye
a brightly lit object has a nice bright reflection, the wood absorbs a small percent
now as that object gets dimmer, and less light bounces to the floor then yout eye
the amount of light absorbed by the wood is a higher percent

get it?:)