We are open to discussion, but I don’t think this is something we would really like to have in Corona, to keep material UI as simple as possible.
Not sure if it’s easier than duplicating the material, but you could use the RaySwitch map (rather than RaySwitchMtl) into the Diffuse in a single material, and give the surface one color for GI and one color for Direct, e.g. by using CoronaColors into the RaySwitch map (would also work for texture maps, so you could desaturate one for GI vs Direct).
E.g. like this set up - at least it means if you wanted to adjust any other parameter, like glossiness or refraction etc, you’d just have to modify that in the one material rather than two 
(or is that what you are doing already? Not sure if the request was to avoid duplicating the whole material, which this does, or if you are already doing this and was just hoping for a way to avoid the RaySwitch map)
