There are two points of possible lubrication being talked about here. The oil in the bevel box is one, and the other is a possible belt dressing. I'd not put oil anywhere near the belt and suspect it would cause the belt to fail.
While the Morgan belt is subject to higher loads than it would be on a bike, more weight and more tyre resistance, the basics of how one looks after a belt should be well known to the motorcycle brigade as they are quite common on Harleys and recent choppers.
