With a cannister, following the pattern of having an internal, could you not just crack open your external, whip out the media baskets and sit them in the tank?
I wonder as well if a simple 'powerhead' would draw less power than the pumphead of the cannister. I'm thinking you could take the baskets out the cannister, sit them in the tank, place the powerhead pointed at the baskets, perhaps also breaking the surface on the 'suction' side so as to 'suck' in some air and 'blow' it through the water, and then just run the powerhead on the UPS?
Agreed that might be a bit of hassel for a 30 minute powercut, but if it was overnight then it sounds like that might let you get some sleep!
Hmmm... to make your UPS last even longer howabout a segment / electronic timer between the UPS output and the powerhead, switching it on for just 10-15 minutes each hour?
Just brainstorming here
With regards to the original UPS problem, perhaps it needs some new batteries, but again, sometimes these batteries are just as expensive (if not more) than a new UPS