In a non-reactive container, combine the soy sauce, sake, oil, garlic and ginger. Place the roast in and seal the container either with a lid or some plastic wrap. Marinate in the refrigerator from 4 hours to overnight.
When you are ready to grill, oil your grids and preheat your grill to 450°F.
Prepare your sweet potatoes by tossing the sliced potatoes with the oil, garlic, onion and salt.
Remove the pork from the marinade, keeping the marinade. Sear the pork roast on all sides, and grill sweet potatoes over direct heat until grill marks appear.
While the roast and potatoes are searing, pour the leftover marinade into a saucepan and heat over medium-high heat until it begins to boil. Reduce the sauce to a simmer and add 1 tbsp. of honey. Allow it to simmer steadily for at least 15 minutes.
When the roast has seared on all sides, turn off one side of the grill so that the potatoes and the roast are both cooking over indirect heat. Grill for another 15 to 20 minutes, or until the potatoes are cooked through and a meat thermometer reads 165°F.
Remove the pork from the grill and slice between each of the bones for serving. Serve with grilled sweet potatoes, seasonal vegetables and some of the sauce on top. Or over a bed of rice.