Make the coleslaw: In a large bowl, whisk the mayonnaise, vinegar, honey, horseradish and celery seed until well combined. Taste and season with salt and pepper. Add the coleslaw mix and toss until well combined. Cover and refrigerate. (You can make this up to 1 day ahead; keep covered and refrigerated. Toss before using.)
Preheat the oven to 400ºF. Line a large baking sheet with parchment. Pierce sweetpotatoes all over with a fork, place on the baking sheet and bake until tender (a paring knife should go in easily), 45 to 50 minutes. Let cool slightly, then cut each in half lengthwise.
Scoop out the sweetpotato flesh with a spoon (save for another use); leaving behind a thin layer of flesh in each skin. Place the skins back on the baking sheet, brush all over with olive oil, season with salt and pepper and bake until the skins crisp up, 12 to 15 minutes.
Meanwhile, heat the carnitas in a large skillet, following the directions on the label. Reduce the heat to medium-low, stir in ½ cup barbecue sauce and cook, stirring occasionally, until the sauce is warmed through. (Add more sauce if the mixture is too dry for your taste.)
Divide the sweetpotato skins among 4 plates. Spoon the carnitas on top of each, dividing evenly. Pile coleslaw on top. Sprinkle with parsley, if using, and serve.