How to ? Simple.
Enter your Map.dbc. Go to the ID of your map. Look for column 64 (ExpansionID). Change the "Expansion" Number for the continent to at least 1 - better 2. Save. Include on serverside and clientside.
Enter your Areatable.dbc. Go to the ID of your area. Make sure you take the region, not subregion flag (e.g. Barrens). Look for column 5 (Flag). Add 0x00004400 to the area you want to enable flying at. Save. Include on serverside and clientside.
Necessarities:
Players and server have the .dbc file with changed flag.
The Map.dbc must have an updated expansion value for the area you want ot fly in.
If you want to limit areas people can fly in, set restrictions (either in Noggit or some other way). You do not dismount when leaving for an area you can not fly in. You just can not re-mount again.
Despite items saying something about "works in area only", you can freely use and activate every mount not restricted to a specific area (example: Item ID 40600 for Bone Gryphon)
May or may not cause issues with custom continents (.wdt may crash).
Players must have expansion 2 enabled to access Azeroth after that - else it may lead to weird errors.
