So, I ended up with a SU1400 Smart-UPS as a castoff from campus network services, and alas, it only has a serial (RS-232) port, not USB, and I’m attaching it to a Mac. So, I went online, and found out that lo, APC makes something to address that (only officially, it only works with the Back-UPS models). $37 later, I had the adapter, and plugged it in. Nothing. Nada. Zilch. Neither Mac nor PC would see the adapter was there. I figured it was dead, and prepared to RMA. But, on a whim, I decided to try putting a smart-signaling cable between it, and the UPS. Viola! My Mac instantly saw there was a UPS there. So, the adapter must be getting power from the serial side, and not the USB side. Weird.
Anyway, thought I should share that piece of knowledge. So, not only do you need the adapter (AP9584), but you need a APC Smart Signaling cable (2 meters 940-0024, or 15 feet 940-1524). Or, just make your own following the info on the apcupsd site.