Theoretically, you can send magic (WOL) packets over WAN, but it doesn't work with MX (according to my tests, if MX has no IP and MAC address binding in ARP table, it drops the packet, it also prevents sending packets over WAN to IP broadcast address). A good solution to this problem would be to enable WoL packet sending via API, but this is not implemented. It stays through the dashboard, but it is not scalable with a network with over 150 branches. 😐
You can try to vote for WoL through the API:
https://community.meraki.com/t5/Developers-APIs/Wake-on-LAN-API/m-p/110831