Isolate low power i2c ADC
The easiest way is to use an isolated 5V:5V DC-DC converter to take the USB supply and provide some power at the other side of the isolation barrier. Probably something like 1W will suffice. 

There will be some noise and some capacitance across the converter (as well as the capacitance across the I2C isolator). You may wish to add additional capacitance across the converter to conduct noise (read the DC-DC converter data as to that, as well as minimum load, maximum capacitance load and other details). 

The common mode range, in theory, would then only be limited by the voltage ratings of the isolators and any capacitors you have added.