So, progress report:
* Optical drive situation was resolved by getting a 12.7mm drive that can also read BD discs, which will be useful.
* new PDB arrived from Supermicro, but... the 24-pin ATX cable is just barely long enough to plug into the motherboard, and with everything installed in the chassis the system won't power up. If I set the PDB and power supplies on top of the chassis and plug in all the cables, as unsafe as that situation is, the system will power up and work as it should (but not with the used PDB, so that will be returned). I'm going to get a 24-pin ATX extension cable to relieve the stress on the connectors/wires. Note that this is *not* an 'S8824' or 'L8824', just a regular '8824' PDB.
* the I2C connection from the PDB to the motherboard is working fine; the BMC reports the details of the power supply units and the power consumption.
* the room is still very cold, so the power supply fans are running at 3500-4000RPM, and they are nearly silent at that speed.
* the BMC reports that peak power consumption so far is 98W, and average is around 70W, and that's with five 4TB spinners in the chassis. Even when the CPU is busy that number likely won't go over 120W.
Can't call this one 'done' until I've sorted out the PDB/power-on situation, but it's getting close to done.