I have just built a bleach bypass post process shader for the UDK. It is based on the material editor node system, so there is no hlsl at all. The performance impact is roughly 10-12% - tested on a low end GT520 gfx card. Material paramters allow to adjust the overall effect intensity, brightness and desaturation.
What the bleach bypass basically does is creating a desaturated copy of the scene and overlaying it (this means screening the bright areas and multiplying the dark ones). This will result in a scene with less saturation and a higher contrast. For further information read trought this: http://en.wikipedia.org/wiki/Bleach_bypass