Check the fillets for pin bones by running a finger along the center line and pull any you find with tweezers. If the fillets are large, cut each one in half crosswise so you have 4 pieces. Pat dry.
Sprinkle the sake over the flesh side, if using, and let it sit 2 minutes, then pat dry again. Salt both sides evenly, going a little heavier on the skin side. Lay the fillets skin side up on a plate or rack and leave them uncovered at room temperature for 20 minutes. Beads of moisture will come to the surface.
Meanwhile set an oven rack about 5 inches below the broiler element and turn the broiler to high. Line a rimmed sheet pan with foil, set a wire rack on top and brush the rack with the oil. If you do not have a rack, oil the foil directly. Grate the daikon and squeeze out the excess liquid so it is damp, not wet.
Pat the fillets dry once more with paper towel. Do not rinse the salt off, just wipe away the surface moisture. Make 2 shallow diagonal slashes through the skin of each piece so it does not curl. Lay them skin side up on the rack.
Broil for 7 to 9 minutes without turning, until the skin is blistered and browned in patches and the flesh at the thickest part flakes when pressed with a fork. Thin fillets can be done in 6 minutes. If the skin is browning faster than the fish is cooking, drop the rack one level lower. There is no need to flip; the rack and the hot foil cook the underside.
Rest 2 minutes on the rack. Serve skin side up with a mound of grated daikon beside each portion, a few drops of soy sauce on the daikon, a lemon wedge and shiso if using. Squeeze the lemon over the fish at the table, not before, or the skin softens.