Sigh. So it looks like "Common Slot" power supplies aren't actually "common slot". The "other" pins are different between models. For my platinum rated power supply, the pins 33 and 36 shorting will cause the power supply to turn on. But for the backplane, it looks like 31 and 34 will turn on the power supply while at the same time pin 32 is hardwired to the 12v. So much for platinum power supply.
This p/s is for the DL385G5P server line, much heavier duty than the p/s that the backplane was designed for (DL380G5)
The good news is that the pins needed to turn this on are exposed already at the back of the p/s connector of the backplane. Just run some light duty (+/- 22awg) wire from those two pins and connect them to an external switch. Or if you you just want the power supply on whenever plugged in to the backplane, just run a small jumper between the two pins.
Please make sure and check the power polarity on the backplane once it's running. You don't know if it's reversed compared to the regular p/s.
With a soldering iron, this should take less than 30 seconds to do.
Cheers
Art