Heat oven to broil with rack set in upper-middle position. Arrange 2 cut buns, cut side up, on a baking sheet and drizzle with olive oil. Broil until lightly toasted, about 1–4 minutes depending on broiler.
Meanwhile, if using mini meatloaf, cut 1 meatloaf in half to create two even slices. If using leftover regular-sized meatloaf, cut two thick slices from loaf. Arrange meatloaf on baking sheet and broil until deeply toasted and edges have crisped, 3–8 minutes, depending on broiler.
In a large mixing bowl, whisk together ½ cup mayonnaise, ½ cup chopped kimchi, and ½ teaspoon garlic powder. Measure out ½ cup and set aside. Add 2 tablespoons lime juice (or vinegar) to remaining mayo mixture and whisk to combine. Add 2½ cups cabbage and 1 cup cilantro, if using, and toss to combine.
Divide reserved mayo between toasted buns. Divide slaw between bottom buns then place broiled meatloaf over top. Place top buns over meatloaf and enjoy.