My idea would be to ditch the 5V, 12V, everything, and replace it with a single 3.3V or 5V AUX (standby rail @ 3A max) and a relatively unregulated 30 to 42V DC rail. Reasoning behind this is 30-42V is within the range of a lot of 42V DC-DCs so there's existing parts base, it's reasonably safe (can't shock yourself on 42V, it might tingle a bit but that's it) and being tolerant of unregulated supplies means you can directly power the PC from a 10 cell lipoly pack or an 18 cell (3x12V) lead acid battery. Also you wouldn't need a ridiculous 80A on 12V for 1kW, but a more sane ~35A. In the ideal design, the aux rail would be optional, and if it is not present, the motherboard provides the aux power from the main power, otherwise the aux power is used to run standby functions and turn on/off the main rail.
The only things that really care about regulation are fans and motors, so there'd either need to be speed regulation on each or a 12V DC-DC on the motherboard.
It could also be practical to power computers off a central DC supply, but it would be more likely to be 300V+... I believe some server rooms are doing this now. Obviously you don't want 300V in your computer.