We already have support for this in every OS, it's no different than a laptop or existing UPSs. What sucks is that to work with a generic ATX motherboard without a special header to hook into the best externally accessible bus to put it on is probably going to be USB. It's not like you would need to plug it in outside the case either, there's usually a spare USB header anyways.
But hypothetically, APC could make an ATX UPS today that would be compatible with most ATX motherboards and leverage most of their existing software and drivers to do so.
I just worked on a PC this weekend, and all the internal USB connectors went to sockets on the front panel. But it should be possible to put a socket on the back of the power supply and include a 1 foot cable. This is doable!
It would be interesting to see the power supply internalized, or externalized with a UPS battery... would probably passthrough/convert, similar to many mITX systems..
But hypothetically, APC could make an ATX UPS today that would be compatible with most ATX motherboards and leverage most of their existing software and drivers to do so.