Thought about that too. From the 24 pin ATX connector I route some of the 12V pins to their own 12V bus. That bus is used to power the motors. Have a second 12V bus for things like fans. Then make a 24V adapter board with regulators on it. It would take 24V in and it would have 24V out for the motor bus, 12V out for the fans, 5V out for whatever, and 3.3V for the ICs.