Multiplicative shadowing for passthrough relighting

Is it at all possible to apply multiplicative effects to the passthrough layer? Currently working on a toon water shader. In this case we need to be able to apply a blue filter to the passthrough for areas that are underwater. Having the effect mesh use a multiplicative shader material has no effect as it appears the passthrough is composed with the scene rendering after the full unity render is complete. I suspect this means it is impossible to do multiplicative blending with the passthrough unless some additional feature exists to explicitly do so. Does anybody know of a way to achieve this effect?