There's no way to turn on clipping in an image other than by what you're doing with extra mask. I'm not sure how you're set up using threshold, but you can create one gradient overlay adjustment layer and adjust the blend if sliders to achieve this result. Make a gradient using blue for the blacks and red for the whites. Then using the blend if sliders in the layer style, switch the highlight and shadow sliders around and set them to the number you want to show clipping.
