Currently if you want to pull UDT data from fortigate devices, it must be done through SNMP. In a router on a stick environment where the fortigate serves as the default gateway for all devices, this essentially forces you to use SNMP monitoring if you want to get useful connected device information (like IP and MAC address) through UDT.
Meraki devices currently allow for pulling UDT data through API. Please look in to implementing this same feature with fortigates.
