I don't think a 12V fan would provide sufficient load on the 5V rail. I think you need a load of several hundred mA. RepRapPro used to supply an ATX PSU with the Ormerod kit. Their ATX power distribution board has 2 resistors, a 30 ohm 5W one and a 22 ohm one that I guess is about 2W. But I can't easily tell which outputs they are loading. My guess (based on the power ratings) is that the 30 ohm resistor is across the 12V rail, and the 22 ohm resistor is on the 5V rail.
One easy solution would be to connect a 12V rear light auto bulb across the 12V supply (AFAIR these bulbs are about 5W), and another across the 5V output.
One easy solution would be to connect a 12V rear light auto bulb across the 12V supply (AFAIR these bulbs are about 5W), and another across the 5V output.