:) Maru. My metod is almost the same thing what u describe for solution, buit litlbit diferent and easyer.
In my metod i do litl trick. Maby is not phisicaly corect but jet, maby it is. Look at this my work in corona , i just did what i described as a solution abowe, not only for corona renderer, but for many renderers.
So look at glass and thell me do u see somthing wrong or not?.