Color2Gray for InDesign for Mac

From Rorohiko:

This tools allows you to convert placed color photos to grayscale without modifying the original color image.

Sometimes you want a particular picture to be output as a grayscale image, yet the original image is in color. You could open it in Photoshop and convert it to grayscale, but you can instead also use this plug-in. The original image is not changed and remains in color, yet it is output as a grayscale image. The gray image will only render on the black (K) plate.