I'll have to have a look when I'm next in the vacinity but I'm pretty sure I have an APC Easy UPS on mine and it works out of the box.
Let me get back to you...
Update: It's an APC Back-UPS 850. No doubt the instructions banged on about requiring Powerchute but I just plugged it into the Syno and it worked fine. You do need to enable UPS support on the NAS itself of course, from Control Panel/Hardware & Power/UPS, and set it to USB UPS.