How difficult would it be to make a variable power supply from an ATX supply that is capable of 20A at 20V?

Very difficult considering your talking about a 400W single output. Most average PSUs only have a total 36A across 2 separate 12V rails. While you might be able to put the 2 rails in series to get 24V you'll only have 18A available. If you wire them in parallel you'll only have 12V but up to 36A available.

If you go with a high end 900+W PSU that has 4 separate 12V rails rated for 18A each than it might work. But the 12V rails need to be truly separate rails. Often PSU manufactures have a single 12V rail that can handle up to 30A or more and call it separate rails. Your best bet would be using 2 different ATX PSUs with a single 20+A 12V rail and wire them in series. Then its a matter of making a good voltage regulator that will support the 20V @ 20A max your looking to get.