I believe that the Postwork Manager is exactly that. It applies postwork settings to a render before you export it. I don't think it actually works during the render process. I think it is like exporting the render, and loading the file into another program to adjust brighteness, contrast, GC, etc..

lmk

The red became pink because the incoming material is not anti-gamma corrected before applying the final gamma correction. If you were to anti-GC everything first, it would work fine.

Post-work GC does not work correctly by itself. Things become desaturated, and hues can shift. Shader GC as I do it works correctly, but has the problem that it isn't compatible with IDL. When IDL is used, the colors coming from shaders must be linear, not sRGB.

However, my HSV GC postwork behaves more like HSV ETM, which means there is less requirement to anti-GC incoming material. But I'm too busy to post it right now.
 

i think you already posted teh HV GC postwork right? we load the render in the background shader.

Yes. You can also put that type of shader on a square (the "Artistic Lens") and render it directly.

Is gamma correcting for print a different science?

I convert P7 renders to grayscale with PS7. 
When I receive proof copies, the gs images can come out pretty terrible.

That depends on what the printer is expecting. But generally printers expect things in sRGB color space, same as your screen, same as your digital camera produces. In other words, if a photo doesn't print to your satisfaction, then a GC'd render in sRGB space will not print well either. If a photo does print well, then a GC'd render should print well.

Converting a color image to grayscale is another topic with its own complications. There are many ways to do the conversion. Two are most common. The first is to pick the brightest value amongst R, G, and B as the grayscale luminance. The other is to calculated a weighted sum of R, G, and B. The latter is how black-and-white TV worked. The factors in B&W TV are:

.3 * R + .59 * G + .11 * B

I'm not sure which techniques PS7 implements.

Here's a nice little page on the topic with respect to the program "the GIMP":

http://gimp-savvy.com/BOOK/index.html?node54.html