Coming back to this discussion after a while… 
I have an idea of how to obtain the intersection points through the dielectric slab without doing a full ray-tree search. What I do is create a “virtual wall” after the slab and trace the rays only up to the slab. At this stage the search is trivial since I know exactly where the rays should end up. Then I use the intersection/directions at the “virtual wall” as an input for another rtrace run, from the wall into the actual scene, but without generating a full ray tree which allows me to do multicore computation. This seems to work, I just wonder if there are any potential pitfalls or innacuracies I have to lookout for?