Yes WOL is possible in Z5xx system. Since Embedded Controller (EC) enables slip exit sequence from S5, you need EC to support this feature. (Power should be available to Ethernet controller & EC during shutdown mode)
Once magic pocket was sent from any host PC, your onboard ethernet controller will send Wakeup signal to the EC. Then EC enables the exit sequence.
Please check Poulsbo datasheet section 18.104.22.168 "S4/S5 to S0 sequence" for more information.
We were able to wake-up the Z5xx system from shutdown through LAN.