Yes that should work. On processor reset, A5 will need to be setoutput to out and low.
Then on setting A5 high AND there is enough power in the LiIon battery the Xbee Radio should come on.
It should be noted its a good idea to monitor the LiIon battery, as it should be sized for the power usage (I like 2Ahr or 4Ahr battery).
When the measured LiIon voltage gets low, it won’t be able to supply the power surge needed to turn on the Xbee Radio.