Remember the capacity of a lead acid battery p l u n g e s to nearly nothing as they get to those temperatures. If you are serious about this, you need to use a much larger rated one and then put it in a temperature controlled box.
You need a fan and a little heater. Normally neither need run but if the space gets too warm because the UPS is running, or battery charging, you turn the fan on and if it gets too cold like under about 35F you turn on the heater. This also lets you use any UPS rather than one that may cost a premium to give you 10% of its capacity at a low temp. The temp control an be very rudimentary since you can have a dead band of about 40 degrees.
Agree with itsmoked 100%. A UPS works as a UPS within its specified ratings. The only big problem is on the batteries! At very low temperatures, you need a good temp-controlled enclosure on the batteries as itsmoked posted.
I don't know if it tested that low, but maybe try a rotary battery instead of chemical. Especially at that low VA the rotary will be huge compared to your load needs, but it's not chemical.
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I run into the same issue when I have to specify emergency and exit lights for large freezers that do not have a backup generator. Luckily I can specify emergency lights with a built in heater in the battery enclosure since, as others have said before, the capacity of a battery drops rapidly with temperature.
The flywheel solution is definitely technically feasible but financially feasible is another question. By the time that you buy a flywhell system you probably could have heated the entire room let alone just the enclosure for the UPS. Depending on the length of time the UPS is expected to run you shouldn't have to worry about it becoming cold in the backup scenario since just operating the batteries will keep them somewhat warm, 90 minutes shouldn't be a problem.
