PaulBags:webwat: Also thought USB support for common UPS might be worthwhile, so router can shut down low-priority services when UPS battery gets low or email the user when UPS battery needs to be replaced. Most UPS now have USB link for updating battery info to a computer.
Is this close enough to your idea?
Yes I just noticed that one, thats what I was thinking as a second option.
Batteries for more that a few seconds could not be internal, however inverting battery voltage to 230VAC and then back to 12VDC into the router seems inefficient. A battery backup that plugs straight into the 12V input of the router would be more ideal and eliminate the temptation to plug all kinds of other 230VAC devices into it, so the battery could be dedicated to the router only. Might need to reinforce the need to put batteries on the ONT though...
No device is 100% failure proof, so when its mission critical you look for redundancy and ways to eliminate every "single point of failure". Power supply is usually the big failure risk, routers need to be rebooted sometimes too.