@mobious, I was thinking something similar, cat cable wired to a USB for the sensor. I’m not a massive electrician but I think so long as the transformer can supply enough amps at the correct volts it should work. I doubt they draw much. I’m not sure what distance you could get on the wire.
CAT5 should be fine unless at some point, you’ll decide to connect a couple of LED strips.
While hands are dirty, I’d probably add some filtering capacitors here and there - who knows what’s on the output of your ‘cheap UPS’.
I use one to feed my multisensor 6 that is mounted in the ceiling of my bathroom. It doesn’t have a UPS, but to me that’s overkill.
I also just ordered a few voltage regulators to make my battery powered sensors work off USB:
I am tired of changing batteries and one of my sensors in the attic is hard to get to so plan to cut a USB wire and feed its power to this regulator and then attach the 3.3v output to the battery terminals. If this works I will convert many other battery only sensors as well.