Toss together cheese, beans, pork rind, and 1/2 teaspoon salt in a large bowl with your hands, then press mixture firmly into 16 (1 1/2-inch) balls (for filling).
Combine tortilla flour, water, and 1/2 teaspoon salt in a large bowl and knead with your hands until a uniform dough forms, about 1 minute. (Dough should be moist but not sticky. If necessary, knead a little more tortilla flour or water into dough.)
Flatten 1/4 cup dough between moistened palms into a 4-inch disk. Wrap disk around a ball of filling, enclosing it, and form into a smooth ball. Reflatten between your palms into a 4-inch disk (1/2 inch thick; filling should remain hidden). Put on a tray lined with plastic wrap and cover with plastic wrap to prevent drying. Make 15 more pupusas.
Heat a comal or large (2-burner) griddle over medium-low heat until hot, at least 2 minutes. Brush lightly with oil, then cook pupusas in batches, turning and pressing lightly with a metal spatula every 2 to 3 minutes, until crusty and browned in spots (some cheese may ooze out), 10 to 12 minutes total per batch. Serve immediately.
· Pupusas can be formed (but not cooked) 2 hours ahead and chilled, covered with plastic wrap.
· Pupusas are best eaten right away but can be kept warm in a 250°F oven while cooking remaining batches. Recrisp on comal 1 minute per side before serving.