Pat the duck very dry. Prick the skin all over with a skewer, taking care not to pierce deeply into the meat.
Pour boiling water over the duck while it sits on a rack, then pat it completely dry. Rub the cavity with five-spice and salt and fill it with the orange and scallions.
Mix the maltose or honey with the boiling water and rice vinegar. Brush a thin layer over the duck and refrigerate uncovered for at least 4 hours or overnight.
Roast:
Heat the oven to 350 F (175 C). Set the duck breast-side up on a roasting rack over a pan.
Roast for about 1 hour 45 minutes to 2 hours, rotating the pan halfway through, until the skin is deeply browned and the thickest thigh reaches 165 F (74 C).
Increase the oven to 425 F (220 C) for the final 10 minutes if the skin needs more crisping. Rest for 15 minutes.
Carve and Serve:
Carve the crisp skin and meat into thin slices. Warm the pancakes according to the package directions.
Spread hoisin sauce on a pancake, add duck, cucumber, and scallions, then fold and serve.
Notes
Overnight uncovered drying gives the skin the best texture.
A rack keeps the duck lifted above the rendered fat so the underside does not steam.
Use store-bought Mandarin pancakes for a practical weeknight version.