You can do that with PixelMath and an ImageContainer.
Just set the image container with all the image you want to process. Apply PM with the equation $T[0] to replace the contents with the red channel. Do the same with all the channels, and the CIE L, changing the equation and the name of the output.
When done, you have to create a new image container, with all the previously generated image, and apply the ConvertToGrayscale process.
Of course, maybe writing an script that suits your needs would be less work for you at the end.