@SteveRichter:

I'd be happy if the standard ATX supplies would have a way of communicating that "oops, main power disconnected, you have about 0-5 seconds to flush data to disk depending on how much juice there is in my capacitors).

Technically it could be made such that it

1) sends a message that mains power is out

2) after certain standardized amount of milliseconds, 12v output is cut, this gives enough time to spinning HDD to decide to say "OK main power cut and no time to write more data to disk, I'm going to try to send this data in my cache back to the OS which can then send it to a 3 or 5 volt flash memory device for store until such time power is restored

3) there could be motherboard input for a battery that is used to keep RAM powered after the PSU caps run out and there was not enough time to flush data to SSD. maybe a driver could be made to write the cpu and RAM to SSD with this battery power at minimal CPU clock speed while GPU's etc buses are powered off

4) apps could request, with user approval, access to the "mains power cut" message, so if user wanted the data of a particularly important app saved with priority over other apps, that could be done.