Let steaks stand at room temperature 1 hour. Meanwhile, combine coffee, cumin, and 1/4 tsp. each salt and pepper. Brush steaks all over with oil and sprinkle all over with remaining salt and pepper. Season evenly on 1 flat side with coffee rub, gently pressing it in (you may have some rub left over).
Prepare a grill for indirect high heat (450° to 550°). For charcoal, leave an area cleared of coals; for gas, turn all burners to high, then turn off one (the indirect heat area). Sear steaks over direct heat, covered, turning once, until browned, 8 to 10 minutes total. Move to indirect heat and cook, covered, until done the way you like, 1 to 2 minutes for medium-rare (130° in center; check with an instant-read thermometer).
Set steaks on a cutting board and let rest about 5 minutes. Drizzle with demi-glace.