Whisk together soy sauce, Shaoxing wine, honey, hoisin sauce, bean curd liquid and preserved curd piece (if using), rice vinegar, garlic, sesame oil, oyster sauce, and Chinese five-spice powder in a medium bowl until smooth. Make the pork

Place pork in a large ziplock plastic bag, and pour in char siu marinade. Press out as much air as possible, and seal bag; shake bag to evenly coat pork in marinade. Place bag with pork on a large plate or rimmed baking sheet, and chill for at least 8 hours or up to 12 hours.

Preheat oven to 425°F. Line a baking sheet with aluminum foil, and place an oven-safe wire rack on top. Remove pork from bag, and place on wire rack; pour marinade into a small saucepan. Sprinkle pork with salt, and let stand at room temperature for 30 minutes.

Meanwhile, bring marinade in saucepan to a boil over medium-high; reduce heat to medium-low, and cook, stirring often, until thickened and reduced to about ⅓ cup, 15 to 20 minutes. Remove from heat. Pour 2 tablespoons of thickened marinade in a small bowl, and set aside for brushing pork. Reserve remaining thickened marinade for serving or for another use.

Roast pork in preheated oven for 15 minutes. Remove pork from oven, and brush all over with about 1 tablespoon thickened marinade. Return pork to oven, and roast until an instant-read thermometer inserted into thickest portion registers 135°F, 7 to 12 minutes, brushing occasionally with remaining thickened marinade (about 1 tablespoon). Remove pork from oven, and transfer to a cutting board. Let rest about 10 minutes before slicing. Serve with reserved thickened marinade and grilled scallions, if desired.