Just so I'm not completely forgetting to put it here too, this is the conclusion of that entire shebang with the OptiPlex GX400 and the rewired PSU adapter.

The good: it works! A newer 24-pin dell supply works fine.
The bad: any supply I shove in here needs to have a 5v rail with a minimum of 22A... And also because I forgot to flip the molex on an inline fan adapter, I scorched the two DVD burners. Spares were on hand but now I'm down to like maybe one more.