How should baked potatoes be reheated in order to most closely approximate fresh-from-the-oven quality?
Sadly, baked potatoes are one of those foods that are only truly at their peak when first baked. Either re-baking them or microwaving them will give the least bad results for reheating.
Your better bet, however, is to give them a new life, for example as potato pancakes, hash browns, as an ingredient in a hash, or similar application. While they won't have the same quality as a fresh baked potato, these can be very good dishes.
Although I agree that they're better off used in hash or some other application, if you really want them as a baked potato, you'll want to reheat them slowly to warm them fully through. You'll need to get them warm enough to re-geletanlize the starches.
I generally put them in an oven near 250°F, wrapped in foil to prevent the skins from drying out further. Exact time is dependant upon the size of the potatoes, but generally about 45min or longer. You can turn the heat up as it gets closer to finished.
I wouldn't suggest this for a dish in which the potato is the main focus, only a supporting player, such as when I'm putting chili or something else over top of it that has a fair bit of moisture and flavor.