While perusing the IMF's data set of Central Bank reserves I did a bit of calculating. The graph below depicts the calculated $ price at which aggregated Central Bank FX reserves could be backed (100%, 35%, and 65%) by their holdings since 1980.
Currently, the IMF figures that Central Banks have 970M oz. of gold and $8,463B worth of FX reserves. 100% cover equates to a price of $8,720.
Alternatively, they could just buy more.