HP ProLiant DL180 G6 refuses to boot
Realized I never answered my own question here. After much troubleshooting, I measued the individual voltage rails and realized +5 V was missing. Turns out the PSU backplane does not just MOSFET-diode-OR the voltages from the power supplies together since the PSUs only produce +12 V to start with and the +5 V buck converter failed the high side FET to open.
The raid rebuild was a disaster. So much data lost...