No, you will not gain Rested XP when servers are down.

This is because they are doing a final save, and then reapplying that save state to ARR.

This means the time from Servers go down, to time ARR goes up 0 minutes will have passed = no rest XP because no time passed in game, no saves, no updates etc.

(also for those that care, Rest XP doesn't build during server maintenance.... just think of this as a really long maint.)