Every Friday and Saturday night from 7:30pm to midnight, Loqui does a taco popup from a window between Pizzeria Delfina and Tartine Bakery. They offer one type of meat each week, listed on their twitter feed or FB page.
Last night, the meat was mesquite grilled beef. I first tried their $3.50 "regular" taco. The corn tortilla is made to order from nixtamal they get at a place in Hayward. The freshly griddled masa has a lovely pliability and a strong enough corn flavor to compete with the mesquite grilled beef. The $5 taco uses a flour tortilla, which seemed handmade, and contains a few extra toppings such as beans.
The wait is about 20 minutes and it's easy to pass the time people watching. Besides, I can't think of a place in the Mission open at that time that has a fresh tortilla that's not made from maseca.