Applying heat is a good idea, except in the cold weather, a heating pad or a hair dryer does not have enough power to warm the metal enough. A heatng pad will not work for the simple reason that it has to be removed to apply the paint and then it can't be re-applied over the fresh paint. The metal will cool rapidly and will need a source of heat until the paint solvent has evaporated.

If you are close enough to an electrical socket, a powerful radiant heater might work. Adjust the distance of the heater from the spot of interest to maintain a temperture above 60 F, but not too hot to cook the finish.

Caution: Most electric heaters for use in the home are not rated for outdoor use (shock hazard). Use only heaters that have adequate grounding and insulation, and protect from splash.

Your hair dryer idea is a good one. Just the warm air blowing over what you touch up ought to do the trick.

Also, FWIW, don't worry about rust due to this. You could touch it up now or later and it won't matter much. When you see vehicles with bubbling paint or holes rusted through due to salt, it happened from the inside out where you wouldn't think about cleaning or touching up, not from a scratch or ding on the surface. Good news is that manufacturers have gotten pretty good at treating metal so this isn't the problem it was in vehicles of say the 70's and before. jmo