Spray a ridged cast-iron stove-top griddle or outdoor grill with nonstick cooking spray and preheat it. Season the scallops lightly with salt and pepper, thread 3 on each side of the skewers, and grill about 1 1/2 minutes per side, or until slightly firm. Remove and set aside.
Meanwhile, place the mustard, vinegar, lemon juice, and a pinch of salt and pepper in a small food processor. With the machine running, add the oils in a stream. Process for about 30 seconds, then add the boiling water and process for about 10 seconds more.
Mix the lettuces with the herbs and the dressing, reserving about 2 teaspoons of dressing. Divide the lettuces evenly among 4 plates. Arrange 3 scallops on each plate, drizzle with the reserved dressing, and serve.