right, if you create an image of the same dimensions as your target image, and set every pixel in that new image to 0.5, and then use the new image as a mask for the target image, you will apply any process to the image at 1/2 strength as long as the mask is applied and active.
i think it got lost upthread, but sander replied that if you go into pixelmath, set the RGB/K expression to simply "0.5" (but without quotes), then in the dropdown set the mode to "create new image" and "greyscale" and drag the triangle onto your image, it will create a new image with all pixels set to 0.5.