I don't own dawnguard so I havn't the foggiest how the sun sundamage actually works.
But there's the http://www.creationkit.com/OnObjectEquipped & http://www.creationkit.com/OnObjectUnequipped_-_Actor events and there's a function http://www.creationkit.com/IsInInterior_-_ObjectReference you can use to check when the player goes outside. so I'd say it probably is possible, someone will just have to fill you in on the dawnguard side of things
- Hypno