Dude this idea has been discussed many times b4 and i know few people who actually using this sort of setup. But the idea it self never took off as some some down sides where found.
First of all UPSs are more expensive in most cases then the DC converters... Some need serial port to be connected to. Others use different voltage than 12volts...
But i guess if i was to choose between Inverter or UPS i would go for UPS for sure!!!
But then again nothing beats DC converters!!!
But hey thanx for sharing your ideas with us