Scott, thanks for the further about the power supply which i'm sure the OP will find helpful.
Me, i don't have a z620 so can't look at the PSU 
But in my limited exposure to HP, via my Z210, i'd agree HP design their workstations in a robust way.
I guess the point of my post, without resorting to looking at spec sheets and other docs, was to provide the OP with a method to reason that what he was doing was reasonable despite the PEG connector not being standards compliant by virtue of a missing pin. I'd say it was this non compliant connectors that was the causing teh confusion and now that should be resolved in his mind.
As to why HP chose to ommit one power pin within the PEG connector is anyones gues as only HP knows why they designed it in that way and it's not a critisism, just what i see as a fact. Reason for design choices are usually not in the public domain.
As for specs, i long for the old days where specs were real specifications of a system and not sales brochures :?