How are you with analogue design, there is no cook book stuff around generally, you need some op amp theory and ohms law (as it applies to electrical and thermal design) and good wiring practice for high power circuits.
Basic principle in enclosed note, you can substitute an NMOS fet, taking into account power supply rail and Vgs. Reverse transistor or fet type (pnp or pmos)for negative supply testing, have to have say +-15V supply rail.
http://www.allaboutcircuits.com/technical-art...r-an-op-amp-output-for-higher-current-part-1/For the higher voltage supplies the sense resistor could be larger to keep the power dissipation on the pass element lower, you have to check for voltage compliance, the voltage across the sense resistor and pass element has to be less than the voltage being tested, the pass element voltage needs to be checked against the data sheet, must not get near Vsat for transisor, or Vds has to be OK for drain current, keeping the pass device in its active state. And so it goes on. Spice may help a bit, but you need to design the whole thing on paper first to understand what you are doing.
If this all seems excessive, then, a lot of variable power resistors may be the answer, if they all had dials, you could set up a spread sheet according the supplies name plate voltages and currents, the results from the spreadsheet could then be dialed up on your test board.
Again, you are talking a hundred watts on some supplies, so these are heavy duty pots, they are available, but very very expensive. Additionally, the power dissipation for a pot is for the full resistance, so the power rating drops as the resistance drops, something a spread sheet could take into account.
Seems to me the active solution offers a lot of advantages in this respect.
If you are in the business of testing a lot of supplies, then the cost may not be an issue, you will have to look at the payback time.
Without knowing more details, it's hard to know what to suggest,
cheers,
Richard
