Lot easier actually, I use this workaround also in V-Ray where refraction at 1.0 makes glass invisible.
Just plug for example CoronaColor set to complete black, or Output map with RGB Level at 0, or Multiply map with both slots set to black (pretty much anything that outputs just plain black color and is not slow) into diffuse slot of your material. You will then have correct black diffuse for metals in render. Now you can simply change your diffuse color swatch in your CoronaMTL to bright gray, to represent metal, but rendering will still use the map in the diffuse slot, and diffuse color swatch in CoronaMTL will be ignored, so everything will work right :)