it wouldn't defeat raytracing o_o raytracing's about accurate light pathing and reflections. adding a filter to the result doesn't change anything about itI see no reason why it couldn't, but it'd kinda-defeat the point of having ray tracing.
The only thing I could see being a 'reasonable' solution is just bringing the black level up to a point where it's more of a dark grey
most toon-shaded media are actively shaded and not simple texture maps
bringing up the black point is also valid too