Ned,
If your UPS sends some sort of command string over a serial line, it easy to
write a process that would parse the string and trigger a shutdown under A/UX.
If the two serial ports of the Mac are already used, you need an additional
serial line interface and an A/UX driver for it. Applied Engineering makes
async multiplexers and A/UX driver software.
You can use the same trick if your UPS sets an output line using a parallel
interface NuBus card. National Instruments makes such cards. I don't know
about A/UX drivers though.
Finally you would have to integrate the shutoff process in /etc/initab so it
would be launched when you boot A/UX.
I have no ideas if you can buy this stuff ready made.
Alberic