Place cornhusks in a large bowl of hot water; weigh down husks with another bowl. Soak for at least 30 minutes. Drain husks; rinse with cold water. Drain and pat dry.
Tear 2 cornhusks lengthwise into 12 (1/2-inch-wide) strips (6 strips per husk).
Preheat oven to 400°.
To prepare filling, trim fat from pork. Combine the cumin and the next 4 ingredients (cumin through paprika); rub pork with 2 1/4 teaspoons cumin mixture. Place pork on a broiler pan coated with cooking spray; insert a meat thermometer into pork. Bake at 400° for 30 minutes or until thermometer registers 155°. Cover pork loosely with foil; let stand 10 minutes. Cut pork in half crosswise; shred with 2 forks.
Heat a small nonstick skillet coated with cooking spray over medium-high heat. Add onion; sauté 5 minutes or until tender. Combine pork, onion, and 2 1/4 teaspoons cumin mixture.
To prepare dough, combine masa harina, paprika, and 1 teaspoon salt in a large bowl. Add water and oil, stirring to form a soft dough. Turn dough out onto a lightly floured surface; knead lightly 4 to 5 times. Divide dough into 12 equal portions. Working with 1 portion, place dough in the center of 1 husk. Press the dough into a 4 x 2 1/2-inch rectangle, leaving about a 1 1/2-inch border at the tapered end and a 1-inch border at the broad end. Arrange about 1/4 cup filling down center of dough. Take 1 long side of husk, and roll dough around filling, making sure dough seals around filling. Tie 1 husk strip around tapered end of husk to secure it. Trim all but about 1/2 inch excess cornhusk from broad end. Repeat procedure with remaining husks, dough, and pork mixture.
Arrange tamales in steamer, and cover with steamer lid. Add water to a large skillet to a depth of 1 inch; bring to a boil. Place steamer in pan; steam half of tamales for 12 minutes or until the husks peel away cleanly. Remove tamales from steamer. Repeat procedure with remaining tamales.