What do you mean fake an hdr? do you mean taking one image and turning it into an hdr image..if so as long as the image is not blown out anywhere and its shot in raw adjust the image 1.5 stops both ways and run it through photomatix. This works well,not as good as separate exposures but decent, this is how the majority of hdrs of people are done considering its tough to get three differently exposed images of a person even when using continuous shooting and bracketing without them moving to some extent. If your not combining images then I would say its pretty tough to create an HDR. You can use a high pass filter on an overlayed layer, that will give your contrast a boost, combine that with mulltiple USM filters and adjustments with the shadow/highlight filter and youll get something the looks unnatural but it will not have that hdr quality to it.
Personally in my opinion HDR is an interesting effect but as more and more people learn to do it correctly and it becomes commonplace I believe it will be nothing more than selective coloring or long exposures with moving lights, they are interesting to a non photograher but not that impressive when shown to someone who knows how you did it.