Would an ATX PSUs internal fan work as the dummy load for a benchtop power supply conversion?
The problem is that most of the output voltages are generated directly from different windings of the same transformer, so it is not possible to regulate them independently. 

If you only load the +12V output the controller will increase power to the transformer to compensate for voltage drop in the rectifiers etc., but this causes voltage on the other unloaded outputs to increase. If the +5V output exceeds its maximum permitted voltage the crowbar will trip and shut the PSU down. 

Loading the +12V output down more will only make it worse. To prevent the +5V output from rising too high you need to draw a reasonable amount of current from it, typically 2-3A.