Trimming: The hardest part to stuffing a pork loin roast is in the carving. Before you slice it open, make sure to remove all the fat and sinew from the surface of the roast. I know you're thinking that the fat will help to keep it moist and tender, but in this instance it will simply make a hard shell and can cause the roast to burn before it gets cooked all the way through.
Butterfly: To stuff the pork loin, you'll need a place to put the stuffing. You can do this in two ways. First, the simplest method is to butterfly the roast. Using a sharp knife cut down through the center of the roast and fold it open like a book. Next, pound the meat into a uniform 1-inch thickness. This will allow you to roll the roast back up once it's stuffed. The alternative is to cut the roast through roll cutting. This technique takes a little more patience (and maybe some practice), but it provides more flexibility and doesn't require pounding to thin out the meat.
Roll Cut: To roll cut a roast, place the trimmed roast on a cutting board so that one end is directly in front of you. Take a very sharp knife and begin cutting on the side, parallel to the cutting board about 1 inch (more or less is up to you) from the board. As you cut, "unroll" the roast. This is kind of like unrolling a roll of paper towels. The great advantage to this technique is that once you've really master it, you can cut a roast to 1/2 inch thickness or less.
Stuffing: At this point you're ready to spread the stuffing over the pork loin. You want to spread an even layer of stuffing, giving the finished product a nice spiral pattern when it is sliced. Roll the pork loin gently so as not to displace the stuffing. Tie it securely with kitchen twine, but not so tight that you squeeze out the filling. Now it is ready for the grill.
Grilling: Generally, a stuffed pork loin is grilled over indirect heat. You need to let the inside cook before the outside gets burned and dried out. A medium heat will be perfect for this. You can place the pork loin over the direct heat to char and caramelize the surface but be careful not to over do it. The pork loin will be done when the pork reaches an internal temperature of 155 to 160 degrees F. (68 to 71 degrees C.). It is always best to let the roast rest before slicing it to let the juices flow back into the meat. About 5 minutes is adequate.
Doneness: One last challenge is checking for doneness. A meat thermometer will read different temperatures from the stuffing than it does from the meat. Your best bet is to check in several locations and take the lowest temperature as the most accurate.
Because pork loin is so lean, you might want to try a couple of tricks to add moisture. Of course, the best way is to use a marinade. One that has an oil base will hold in the moisture and keep the surface of the meat from drying out. Be careful not to let it burn. Another way is the wrap strips of bacon around the outside of the roast. Since you are tying it up with string anyway, why not place some bacon strips under the strings. It will add flavor and protect the outside of the pork loin roast.