[bailey]

I am very interested in why you want to do that, there is methods for doing that, and any load on the 12 volt supply will not affect the other voltages as they are seperate sources and have there own regulators,
why do you want to backward engeneer the power supply ?

as I stated in a earlear post, just place a few 100 watt 12 volt light bulbs on the 12 volt line and when you add one more than it can handel try a 50 watt and so on. you will soon find out where the limit is, this is the least expensive way to do that.
the other method is to buy a lab load simulator, it will be as variable as you want but is very expensive to go that route.
to use resistors you will need some high wattage precision units and they are also very expensive, unless you had a need to do this in a lab enviroment, I would not recommend that you even try messing around with it.
I am lucky enough to have this type of equipment to do this type of work, and I ain't cheap eather.