Here is the API call you can make from a script to do this.
https://developer.cisco.com/meraki/api-v1/create-device-live-tools-wake-on-lan/
Sample code in Python:
serial = 'Q2QN-9J8L-SLPD'
vlan_id = 12
mac = '00:11:22:33:44:55'
response = dashboard.devices.createDeviceLiveToolsWakeOnLan(
serial, vlan_id, mac,
callback={'url': 'https://webhook.site/28efa24e-f830-4d9f-a12b-fbb9e5035031', 'sharedSecret': 'secret', 'httpServer': {'id': 'aHR0cHM6Ly93d3cuZXhhbXBsZS5jb20vd2ViaG9va3M='}, 'payloadTemplate': {'id': 'wpt_2100'}}
)
print(response)
Note that the callback URL is optional - this just notifies you once it has been done.
I haven't tried this with the Meraki API, but with other systems you can specify a MAC address of ff:ff:ff:ff:ff:ff to send the WOL packet to every client in the VLAN at once, with a single call.