I think I have managed to get what you need. :)
Create a Corona Legacy Material (because it's easier to get non-physical properties with it, such as lack of reflections).
Set its refraction to 1 and IOR to 1. That should make it "invisible".
Under advanced options set alpha to "always black".
Then plug a Corona Rayswtich Map into this material's refraction color and make it GI - black, reflect - black, refract - white, direct - white.
It should also work with the Corona Physical Mtl somehow, but I haven't tried it.