Be kinda hard to determine or maintain consistant temps with a hair dryer style heat gun I would think. VHT isn't cheap, (quality never is) as I recall it was $7.00 a can, for that kind of money, why not spray the entire tank? Make it a project for a rainy day. Or during winter downtime.Being that it's just a quarter-sized bare patch, if I used this kind of enamel do you think I could cure it with a heat gun? Yes, I would drain and air-out the tank and line.
But if all you want to do is patch a bald spot, why not try Map's suggestion of using Autozone Lacquer, it is cheaper and wouldn't require the time, effort or expense that my approach does. If nothing else, it may tide you over through the rest of the year until it becomes too cold to ride, thus giving you the time to do it up right.
More than likely it is the cap vent dribbling fuel outside the tank, its very common on these China Girls.