In a blender, combine the achiote paste, chiles, canning sauce, beer and oil. Blend until smooth. Use 1/3 of the marinade to smear over both sides of each piece of meat (refrigerate the rest of the marinade to use on other meat or fish). Cover and refrigerate for at least an hour.
Light a charcoal fire and let the coals burn until covered with gray ash but still very hot; bank the coals to one side and set the grill grate in place. Or, heat one side of a gas grill to high. Brush both sides of the onions slices with oil and sprinkle with salt. Lay in a single layer on the hot side of the grill. When richly browned, usually just about a minute, flip and brown the other side; move to the cool side of the grill to finish softening to grilled-onion sweetness.
Oil and grill the pineapple in the same way.
Finally, in batches, grill the meat: it’ll take about a minute per side as well. As the meat is done, transfer it to a cutting board and chop it up (between 1/4- and 1/2-inch pieces). Scoop into a skillet and set over the grill to keep the meat warm. Chop the onion and pineapple into small pieces as well, add them to the skillet and toss everything together. Taste and season with salt, usually about 1/2 teaspoon.
Serve pork mixture with the tortillas and salsa for your guests to make soft tacos.