Should be fine with a line voltage UPS. Some have had problems with a USB UPS.
Most installations use one AC reference on one of the legs for both legs, but IoTaWatt is very adaptable. It can have as many as three voltage references. So if you want to add another AC reference for the other side of a spliut phase, you can do that. Another approach, which I consider the best, is to use a 240V reference transformer calibrated down by half. See this discussion:
I would not recommend plugging the voltage reference into a UPS.