There's no way to evaluate serial number from any devices AFAIK. Apple also restriction of specific set data with iOS7.
Get ( PersistentID )
is the way to go.
This function gets a unique ID from every device (iPhone, iPad, Mac, PC).
Be aware that the PersistentID on any iOS-Devices will change as soon as FMGo iApp is deleted AND re-installed.
Check the 'Report a Issue' forum for some issues regarding "PersistentID"
You can use the "Get (PersistentID)" function when you register an iPad and store the value in your hardware table. Just run the same function again during your login / initialisation script and try to match it to the hardware table.
You just need to be careful with the iOS devices , if you restore a backup from an iOS device to another, the persistentID goes with it as well.
Edit: efficientbizz was faster than me :-)