It might be possible but I am not sure. I would suggest going the route of using global material parameters. Then you could make a material function that takes as input “current roughness” and applies the global wetness value in some way, outputting the modified roughness. True you would have to apply it to every material and it wouldn’t be as convenient as a screenspace effect, but you would also have far more power to affect how the wetness works on different assets.
Being a global parameter value it would be fairly cheap that way. The parameter being in the function means the function is easy to hook up as no parameters have to be added for each material.