I've just recieved a Rescue Mission to get Kust currently captive in Fort Fellhammer back to Falkreath.

Normally I'd walk him back but this is going to be a major pain at 56th level with ancient dragons turning up all the time.

Is it possible to use fast travel once I've rescued him from the fort?

I'm not sure. What I think I'd do is make a save and try it. If the Falkreath end works okay and the hostage says thanks/bye the way he should, I'd be pretty confident the quest giver end to collect the reward should work fine as well. If it doesn't work out, then you can always reload.

Yea you can fast travel back to the town he's from. He'll be right there with you when the town loads.

Thanks

I've always done it the old-fashioned way and walked them back before but this type of quest I've always done at much lower level and nowhere near as long a journey

Last time I did a rescue quest Sven attacked a dragon with his fists but later ran away from a wolf