Marinate the Seitan:
In a bowl, whisk together the achiote paste, guajillo chili powder, minced garlic, pineapple juice, white vinegar, oregano, cumin, and salt.
Add the seitan slices and ensure they are well-coated with the marinade. Let them marinate for at least 1 hour, or overnight in the refrigerator for more depth of flavor.

Cook the Seitan:
Heat a skillet over medium-high heat. Remove the seitan from the marinade and cook until the pieces are nicely charred and cooked through, about 3-4 minutes per side. Add pineapple chunks during the last few minutes of cooking to caramelize slightly.

Prepare the Tacos:
Warm the corn tortillas in a dry skillet or microwave until pliable.
Build the tacos by placing a few pieces of seitan and pineapple on each tortilla. Top with sliced red onion and chopped cilantro.

Serve:
Serve the tacos with lime wedges and your choice of salsa. Encourage diners to squeeze lime juice over their tacos before eating.