It's a little radical, but I'm thinking of going this route:
-use the MOSFET in its normal "forward" fashion, the way it was meant to.
-In order to prevent any current from feeding back into a PSU that is off (sinceit's not an issue when all PSUs are On), I would add a power diode.
Of course at this point, it's actually a step backwards, but...
-add a relay, to bypass the power diode, where the relay would be powered by each PSU's "Pwr_Good" line. This would be done to get around the voltage drop of the power diode, in its normal forward direction.
The problem I'm afraid, is finding a relay with contacts that have a minimal resistance, and that can handle the potentially high current load.
The relay would otherwise be wired, just like pHaestus did, for wiring a pump to a PSU.
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