I remember someone on here mentioning it ages ago, It sounded pretty complex (it involved using parts from other cars i think), but I tried searching for the original post post and came up with nothing. I'm sure there's some info out there, just have a dig.
You might be able to achive something similar, however, without ever involving OBDII. I'm sure you'd be able to program a PIC microcontroller to convert the values from various sensors in your car (speed, oil temp, boost pressure etc.) into digital form that is understandable by your PC. This would be easier than trying to emulate OBDII IMO.