Prepare the grill for direct grilling with a hot fire, about 500ºF.
Brush the corn and the onion with olive oil and liberally season with salt. Grill the onion over direct heat until blackened on the cut side and the whole onion is soft. Grill the corn quickly, turning frequently, just until marked all around. Remove both from the grill and let cool for at least 10 minutes.
Cut the corn from the cob and loosely dice the onion. Reserve.
Place 1/2 of the lime juice in a large bowl along with 1/2 teaspoon salt and the garlic. Pit the avocadoes and add the meat to the bowl. Stirring as you add each piece in order to coat with lime juice and discourage oxidation. Use a potato masher to break down the avocadoes. Leave it chunky and loose. Stir in the onion, corn, cilantro, jalapeno and bacon. Test for taste, adding more lime juice and salt as needed. Fold in the tomatoes.
If not serving immediately, press plastic wrap into the surface of the guacamole to keep air from making contact. This will keep in the refrigerator for up to 2 hours, but it is best served immediately upon assembly.
Working ahead: The corn and onion can be grilled and the tomatoes can be cut 2 hours ahead of time. The bacon can be cooked up to a day in advance.