Diode rating for a UPS project with extra battery
I doubt this will work as I'd expect the inverter circuit expects the battery to be a very low impedance in both directions.

To size the diode you might take the output power that your UPS can provide and refactor that to the 12V battery.  Assume some loss in transform, say 50%.

If your UPS is rated for 100 watts, then your battery will need to supply 150 watts, about 13A. 

If your idea would work, the diode needs to be about 20A for a 100W output. 

The diode will likely need a heat sink as it would be dissipating about 10 watts.