HighKey is a Photoshop plug-in filter. It looks at each pixel in the selection and manipulates the bits in it.
HighKey looks at each component byte of each pixel (three components for RGB, one for greyscale). If the byte is non-zero, it is shifted left until the leftmost bit is 1, so the byte 00011111 would be shifted left three times, giving 11111000.
If the Maintain Colour checkbox is checked, then for RGB pixels, the components are only shifted left until one of them has the leftmost bit as 1.
Copy the filter to the Photoshop Plug-ins folder to use. I’ve supplied PowerPC (PPC) and 680x0 (68K) versions.