Step 1: Place the juniper berries, peppercorns, and rosemary in a large nonreactive bowl and crush them with the back of a wooden spoon to release the aromatic oils. Add the onion, garlic, lemon zest, lemon juice, wine, olive oil, and coarse salt to taste (about 1 teaspoon). Whisk to mix. Set the marinade aside.
Step 2: Trim any silverskin from the elk loin. Add to the bowl with the marinade, turning to coat evenly. (You can also marinate the elk in a large zip-top type plastic bag.) Let the meat marinate in the refrigerator, covered, for at least 4 hours, or as long as overnight.
Step 3: Drain the elk, reserving the marinade, and pat dry with paper towels. Reseason with salt and freshly ground black pepper. Wrap the loin in bacon strips, using toothpicks to secure the bacon.
Step 4: Meanwhile, bring the marinade to a boil over high heat and let boil briskly until the flavors are concentrated, about 3 minutes, skimming any scum that forms on the top. Let cool.
Step 5: Set up the grill for direct grilling and preheat to medium-high. (Leave one part of the grill unlit to use as a safety zone if needed.)
Step 6: When ready to cook, brush and oil the grill grate. Place the elk loin on the hot grate and grill, turning with tongs, until cooked to medium-rare (4 to 5 minutes per side), cooking on all 4 sides, about 16 to 20 minutes in all. Baste often with the reserved marinade. (The internal temperature, when read on an instant-read meat thermometer, should be about 145 degrees for medium-rare.) If flare-ups occur, move the meat to the cooler part of the grill.
Step 7: Transfer the meat to a platter or cutting board; let rest 3 minutes before carving and serving.