Gordon and Joe's feedback is the best advice on this, but if your budget supports it, this company may have something close to what you are looking for. I stumbled across them a while back when I went looking for the same thing: http://www.roboticsgroup.com/products.asp?id=22 I have yet to buy one & use it but it's still nice to know a canned solution does exist if that's the route you want to go.
Look down the page at their "RGi-PowerCommander Triple Step-Down Programmable Power Supply".
Best of luck ! JCD