In the event of an accident in the carrier, I'd be inclined to say that lining the lower surface with at least 3 layers of puppy training pads and carrying some some small garbage bags. That way if there is an accident, you can slide the used pad out, put it in a bag and dispose of it in the lavatory garbage.

To follow Concorde's very good suggestion of tiring him out, I'd find a grassy area near the terminal and make sure he stays out long enough and gets a little exercise to make sure that anything that might come out in flight comes out while you're on the ground and can be dealt with a little more easily.

Provided that he's house broken, 5 hours shouldn't be too long to be confined.

As to letting him out of the carrier while in flight, if he's as small as you're saying and provided he's well behaved enough to sit on your lap, I don't think the flight attendants will have a problem with you removing him from the carrier in flight as long as you've got a leash on him and can get him back into the carrier easily in an emergency or if he begins to make a nuisance of himself. (Trust me, as soon as I tell anyone that my dog is well behaved, she seems to go out of her way to forget everything I've taught her about manners.)

Consult with your vet about tranquilizing the little guy. In the instance that you may need to, a small portion of a baby aspirin will probably do the trick. I'd ask the vet to do the measuring for you so that you don't make a mistake and accidentally overdose him.

The only thing that's kept my dog from going with me in aircraft before has been the clean up factor. She sheds like nothing you've ever seen before and by the time I'm done cleaning all the fur out of the plane, I might as well have driven there in the first place. However, I know plenty of poeple that routinely take their dogs with them in the cabin of private aircraft. A freighter pilot I knew used to take his dog with him in the company Learjets on an almost routine basis. Everyone I've talked to that flies with their pets, usually unconstrained, in the cabin has said that dogs tend to make better passengers than small children do.

Have a good trip.

JT