I already found the USB pinout for the RJ45-10 connector on my APC BackUPS ES BE500U-CN UPS, and I constructed a temporary adaptor which works. APC's list of cables implies only USB is supported, but before making the permanent cable, I wanted to see what else is accessible via that connector and maybe find a way to do remote on/off.
Dumb (voltage level) signalling is present. When unplugged, pin 2 goes to just over 12 volts, and applying 12 volts to pin 8 for over a second causes the UPS to shut off with a single short beep one minute later. I didn't test the low battery signal which is supposed to be on pin 3.
Smart (serial communication) signalling doesn't seem to be present. Sending Y at 2400 baud 8,n,1 does not do anything. This agrees with what I've read online about it not working.
I wonder what pins 5 and 6 are for. None of these signalling methods use them and there is no voltage or resistance to ground at them. Maybe they're not used.